Fixed and refactored roundtrip, n3_suite and nt_suite tests

[aucampia]

Fixed and refactored roundtrip, n3_suite and nt_suite tests

Issues in previous roundtrip that are fixed:

1. Roundtrip tests were skipping tests that would fail instead of
   reporting them with xfail.
   While this is maybe a minor distinction it
   is more appropriate to report as xfail as that is a better semantic
   description of a known issue. Skipping should be reserved for tests
   that are expected to succeed when ran but can only run in a suitable
   environment (e.g. if berkleydb is installed). Furthermore, marking
   tests with xfail will still run the test, and will also report them
   as unexpected pass (xpass) if they do actually pass.

2. Removed ("json-ld", "example-misc.n3") from xfail/skip as it does
   pass.

Refactoring and improvements:

1. Removed test/test_n3_suite.py::test_n3_writing.
   This test was doing something functionally equivalent to what
   test/test_roundtrip.py::test_n3 would do, except it was doing it
   with different code (test/testutils.py::check_serialize_parse).
   Specifically it would test, parse from n3, serialize to n3,
   parse from n3, and this is also something that
   test/test_roundtrip.py::test_n3 does. One difference is that
   check_serialize_parse was using crapCompare instead of
   rdflib.compare.isomorphic, and crapCompare does a significantly
   worse job but also hides an issue with rdflib.compare.isomorphic
   and quoted graphs which is now correctly reported as xfail.
2. Removed _get_test_files_formats and all_{n3,nt}_files from
   test/test_{nt,n3}_suite.py (and removed the files since there is
   nothing else in them of value). These are now replaced with code
   inside test/test_roundtrip.py which has less duplication and uses
   guess_format for determinering formats instead of a custom if/elif
   block.
3. Change skipping/xfail to work on file basename instead of path.
4. Changed file collector from a generator to a function returning a
   list as the result of this function is iteratred over multiple times.
5. Changed test id's to be more meningful, e.g. roundtrip_n3-writer-test-28.n3_n3_trig
   instead of roundtrip-args303.

Other changes:

1. Renamed test/nt/test.ntriples -> test/nt/test.nt: .ntriples is not
   a recognized extension and the test this is being used in does not
   test anything pertaining to extention handling.
2. Add guess_format tests for .{rdf,nt,n3} and negative tests for
   .{docx,mkv} and a file with no extension.

[aucampia]

Diff in coverage:

  Module                                                                                            | statements | missing | excluded | branches | partial | coverage 
1c1
< # Coverage report: 81%
---
> # Coverage report: 82%
13c13
< [rdflib/compare.py](d_9b73ddaf1c733d26_compare_py.html)                                           | 336        | 18      | 0        | 183      | 11      | 93%      
---
> [rdflib/compare.py](d_9b73ddaf1c733d26_compare_py.html)                                           | 336        | 18      | 0        | 183      | 10      | 93%      
23c23
< [rdflib/graph.py](d_9b73ddaf1c733d26_graph_py.html)                                               | 910        | 178     | 2        | 424      | 42      | 79%      
---
> [rdflib/graph.py](d_9b73ddaf1c733d26_graph_py.html)                                               | 910        | 177     | 2        | 424      | 42      | 79%      
51c51
< [rdflib/namespace/__init__.py](d_d11ffb0fa4573d19___init___py.html)                               | 379        | 33      | 0        | 156      | 15      | 90%      
---
> [rdflib/namespace/__init__.py](d_d11ffb0fa4573d19___init___py.html)                               | 379        | 30      | 0        | 156      | 15      | 90%      
68c68
< [rdflib/plugins/serializers/jsonld.py](d_07086a33ade23e2f_jsonld_py.html)                         | 227        | 17      | 0        | 149      | 17      | 90%      
---
> [rdflib/plugins/serializers/jsonld.py](d_07086a33ade23e2f_jsonld_py.html)                         | 227        | 17      | 0        | 149      | 16      | 90%      
73c73
< [rdflib/plugins/serializers/rdfxml.py](d_07086a33ade23e2f_rdfxml_py.html)                         | 211        | 12      | 0        | 116      | 14      | 92%      
---
> [rdflib/plugins/serializers/rdfxml.py](d_07086a33ade23e2f_rdfxml_py.html)                         | 211        | 13      | 0        | 116      | 15      | 91%      
76c76
< [rdflib/plugins/serializers/turtle.py](d_07086a33ade23e2f_turtle_py.html)                         | 289        | 24      | 0        | 124      | 10      | 89%      
---
> [rdflib/plugins/serializers/turtle.py](d_07086a33ade23e2f_turtle_py.html)                         | 289        | 23      | 0        | 124      | 9       | 90%      
88c88
< [rdflib/plugins/sparql/evaluate.py](d_b85a33f12b1281b4_evaluate_py.html)                          | 315        | 18      | 0        | 193      | 13      | 94%      
---
> [rdflib/plugins/sparql/evaluate.py](d_b85a33f12b1281b4_evaluate_py.html)                          | 315        | 17      | 0        | 193      | 12      | 94%      
116c116
< [rdflib/term.py](d_9b73ddaf1c733d26_term_py.html)                                                 | 697        | 127     | 3        | 379      | 39      | 79%      
---
> [rdflib/term.py](d_9b73ddaf1c733d26_term_py.html)                                                 | 697        | 125     | 3        | 379      | 39      | 79%      
126c126
< Total                                                                                             | 20297      | 2994    | 127      | 7271     | 761     | 81%      
---
> Total                                                                                             | 20297      | 2987    | 127      | 7271     | 758     | 82%      

generated with:

# executed in each dir
.venv/bin/python3 -m pytest --cov-report html --cov

# diff
html2text -b 999 --pad-tables htmlcov/index.html | \
  sed -n 's/^.*statements.*missing.*$/  &/gp' ; \
  diff \
    <(html2text -b 999 --pad-tables ../rdflib.master/htmlcov/index.html | grep -v 'created at') \
    <(html2text -b 999 --pad-tables htmlcov/index.html | grep -v 'created at')

[nicholascar]

Glad @aucampia got rid of crapCompare! I've seen it in there for a while and figured that isomorphic must have come along much later.

[nicholascar]

Merging after one review as fair straightforward PR