globbing: improve test coverage.

[quasicomputational]

For efficiency reasons, matchDirFileGlob isn't a simple call to
getDirectoryContentsRecursive and then a filter with
fileGlobMatches. So test both that naive approach and the actual
approach to make sure they are both correct.

I only realised this was a problem right after #5284 was merged!

Since this is tests-only, I'll merge once Travis and AppVeyor are green.

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

[23Skidoo]

Looks good, thanks!