tests: fix overeager scrubbing of environment variables

tests: fix overeager scrubbing of environment variables

In commit 95a1d12e9b9f ("tests: scrub environment of GIT_* variables") all
environment variables starting with "GIT_" were unset for the tests using
a perl script rather than unsetting them one by one. Only three exceptions
were made to make them work as before: "GIT_TRACE*", "GIT_DEBUG*" and
"GIT_USE_LOOKUP".

Unfortunately some environment variables used by the test framework itself
were not added to the exceptions and thus stopped working when given
before the make command instead of after it. Those are:

- GIT_NOTES_TIMING_TESTS
- GIT_PATCHID_TIMING_TESTS
- GIT_PROVE_OPTS
- GIT_REMOTE_SVN_TEST_BIG_FILES
- GIT_SKIP_TESTS
- GIT_TEST*
- GIT_VALGRIND_OPTIONS

I noticed that when skipping a test the way I was used to suddenly failed:

GIT_SKIP_TESTS='t1234' GIT_TEST_OPTS='--root=/dev/shm' make -j10 test

This should work according to t/README, but didn't anymore, so let's fix
that by adding them to the exception list. And to avoid having a long
regexp put the exceptions in a separate variable using nicer formatting.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Thanks-to: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>