TST: archive: Skip tests that depend on p7zip

All of the checks of the test generator fail on a system without
p7zip.  Before dataladgh-4041 (in particular, before beb5d04), the subset
of checks that existed at that time was skipped with (see dataladgh-3176):

    SKIP: cmd:7z is missing. (Not) Funny enough but ATM we need p7zip
    installation to handle .gz files extraction 'correctly'

However, with the change to the archive file name in 05e9353 (TST:
Expand test coverage to additional relevant compression formats,
2020-01-17), the MissingExternalDependency exception that led to the
skip for gzip formats is no longer triggered.  And even if it were,
the other file formats tested would fail.

Mark these tests as known failures when p7zip isn't available.

datalad/tests/test_archives.py

@@ -22,6 +22,7 @@
     ok_generator,
     OBSCURE_FILENAME,
     SkipTest,
+    skip_if,
 )
 
 from datalad.dochelpers import exc_str
@@ -33,6 +34,7 @@
 )
 from datalad.support.archive_utils_patool import unixify_path
 from datalad.support.exceptions import MissingExternalDependency
+from datalad.support.external_versions import external_versions
 from datalad.support import path as op
 
 
@@ -131,6 +133,8 @@ def test_compress_dir():
 _filename = 'fi le.dat'
 
 
+@skip_if("cmd:7z" not in external_versions,
+         msg="Known to fail if p7zip is not installed")
 @with_tree(((_filename, 'content'),))
 @with_tempfile()
 def check_compress_file(ext, annex, path, name):