test: Rename GIT_SKIP_TESTS to NOTMUCH_SKIP_TESTS

test: Rename GIT_SKIP_TESTS to NOTMUCH_SKIP_TESTS

By scanning test-lib.sh for occurrences of "git" or "GIT", I found
that most of those are internal things, (like the GIT_TEST_TEE_STARTED
variable). But GIT_SKIP_TESTS is part of the user-interface to the
test suite, so we rename it to reference notmuch rather than git.

Also, the GIT_TRACE warning is git-specific, so we drop that as well.