[PATCH] Allow TAP tests to be run individually

Per prior discussion, this small Makefile change allows anything that
uses $(prove_check) or $(prove_installcheck) to have the list of tests
overridden by passing a PROVE_TESTS as a list of space-separated test
file paths.

Particularly handy for src/test/recovery when you want to run
t/009_something_new.pl without running 001 through 008 each and every
time. The current workaround is deleting the tests you don't want,
which dirties up your git tree and is really annoying when checking
out and rebasing etc.

Please backpatch to at least 9.6 since it's trivial and we seem to be
doing that for TAP. 9.5 and 9.4 would be nice too :)

--
Craig Ringer http://www.2ndQuadrant.com/
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