Add a utility for finding test fixtures that allows it to not exist i…

[mhammond]

…n installed dirs

[mhammond]

@kxrob pointed out in #1863 that finding fixtures was a footgun, so I improved it and split it out. Combined with #1879, in CI we previously saw:

SKIPPED: 13 tests - (-2147221005, 'Invalid class string', None, None)
SKIPPED: 3 tests - (-2005401480, 'OLE error 0x88780078', None, None)
SKIPPED: 1 tests - Can't find test .wav file to play

But now it does find the test file, but can't initialize a sound device (DSERR_NODRIVER), so skips like most other ds tests.

SKIPPED: 13 tests - (-2147221005, 'Invalid class string', None, None)
SKIPPED: 4 tests - (-2005401480, 'OLE error 0x88780078', None, None)

(I've split this from #1879 to ensure it works OK when tests are run directly from the installed dir)