4.0 | Tests: flag tests which would lead to fixes, but are missing a `fixed` test case file

[jrfnl]

Inspired by a remark from @fredden in #289 and related to #25, the PHPCS native AbstractSniffUnitTest test case should flag sniff test case files which would lead to fixes, but for which no .fixed file is available to verify the fixes against.

As there are numerous external standards which use the PHPCS native test framework for their own sniff tests, this new failure message should go into PHPCS 4.0 as it is likely to cause test failures for external standards, which may be missing .fixed files for some of their sniff tests.

Also note that the same applies to PHPCS itself (currently missing 5 .fixed files). Issue #299 has been opened to address these and will need to be resolved before the fix for this issue can be merged.

[jrfnl]

Note: I have already prepared a fix for this, but won't pull it until #299 has been addressed.

Also note that #25 may warrant changes to the fix currently prepared.