[ticket/12071] Skip tests that depend on fileinfo and fix expected results

Tests that depend on fileinfo being enabled will now be skipped to prevent
fatal errors while running the test suite. The expected results will be
modified to "application/octet-stream" in test_guess_files() if only the
extension_guesser is available due to the non-existing extension of those
files.

PHPBB3-12071
[ticket/12071] Get rid of unneeded cast to boolean in tests

PHPBB3-12071
[ticket/12071] Add test that covers not available fileinfo

The newly added test case will also emulate a non-existing fileinfo
in order to check if the mimetype guesser is properly working when
fileinfo is not available.

PHPBB3-12071
Merge remote-tracking branch 'github-marc1706/ticket/12071' into develop-ascraeus

* github-marc1706/ticket/12071:
  [ticket/12071] Add test that covers not available fileinfo
  [ticket/12071] Get rid of unneeded cast to boolean in tests
  [ticket/12071] Skip tests that depend on fileinfo and fix expected results