test-lib: tests skipped by GIT_SKIP_TESTS say so

We used to show "(missing )" next to tests skipped because they are
specified in GIT_SKIP_TESTS.  Use "(matched by GIT_SKIP_TESTS)" instead.
test-lib: GIT_TEST_ONLY to run only specific tests

This is a counterpart to GIT_SKIP_TESTS.  Mostly useful when debugging.