warn on incorrectly specified test options in rebar.config

[talentdeficit]

when ct_opts, eunit_tests, eunit_first_files, ct_first_files, erl_first_files,
eunit_compile_opts, ct_compile_opts and erl_opts have values that are single non-list
terms warn and try wrapping them in a list when processing them in the eunit and ct
providers

before:

$ DEBUG=1 rebar3 ct
===> error: function_clause [{lists,sort,
                                     [#Fun<rebar_utils.1.98462181>,export_all],
                                     [{file,"lists.erl"},{line,963}]},
                                    {rebar_utils,tup_umerge,2,
                                     [{file,
                                       "/home/travis/build/rebar/rebar3/_build/default/lib/rebar/src/rebar_utils.erl"},
                                      {line,281}]},
                                    {dict,update_bkt,4,
                                     [{file,"dict.erl"},{line,319}]},
                                    {dict,update_bkt,4,
                                     [{file,"dict.erl"},{line,321}]},
                                    {dict,on_bucket,3,
                                     [{file,"dict.erl"},{line,409}]},
                                    {dict,update,4,
                                     [{file,"dict.erl"},{line,314}]},
                                    {dict,fold_bucket,3,
                                     [{file,"dict.erl"},{line,437}]},
                                    {dict,fold_seg,4,
                                     [{file,"dict.erl"},{line,433}]}]
===> Failed creating providers. Run with DEBUG=1 for stacktrace.

after:

$ rebar3 ct
===> Verifying dependencies...
===> Value `export_all' of option `ct_opts' is not a list, trying to adjust and continue
===> Compiling rebar
===> Running Common Test suites...

[ferd]

I'd rather error out entirely than start mistakenly supporting half-broken configs. After that we commit to doing it for good, I'm not sure I like that entirely? I'm fully on board with the bad config detection though.

[tsloughter]

+1 to erroring

[talentdeficit]

i'll change them to errors. needs significantly more work though. tomorrow

[talentdeficit]

updated to error instead of warn. also changed a few older warnings (ct's test_spec and auto_compile) to errors for consistency

[tsloughter]

+1