tests: make use of our custom filesystem assertions for conciseness

[wisp3rwind]

Follow-up to #4830, #4848 and #4849:

I've had #3690 in the pipeline for a long time; and it has grown too big to look at in one piece. This is just the first commit rebased on master. It is still quite big, but only touches test code, so should be safe (assuming CI is green).

this replaces assertions of the form

self.assertTrue(os.path.exists(syspath(path)))

by

self.assertExists(path)

which includes the syspath conversion and is much easier to read.

Occurences where located using

git grep -E 'assert(True|False).*(isdir|isfile|exist)'

[wisp3rwind]

I'll just go ahead on this one and merge; given that it is a fairly straightforward refactoring of tests, there seems little need for a thorough review.

[sampsyo]

Yay; very good call!