remote-mediawiki tests: annotate failing tests

remote-mediawiki tests: annotate failing tests

These tests consistently fail for me, and were failing before any of
the changes in this series. As noted in [1] there are some known
intermittent test failures. Let's mark these as failing so we can have
an otherwise passing test suite.

We need to add an extra test_path_is_file() here because since
d572f52a64 ("test_cmp: diagnose incorrect arguments", 2020-08-09)
test_cmp has errored out with a BUG if one of the test arguments
doesn't exist, without that the test would still fail even without
test_expect_failure().

1. https://github.com/Git-Mediawiki/Git-Mediawiki/issues/56

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>