t/test-lib: allow skipping leak checks for passing tests

With `GIT_TEST_PASSING_SANITIZE_LEAK=check`, one can double check whether a memory leak fix caused some test suites to become leak free. This is done by running all tests with the leak checker enabled. If a test suite does not declare `TEST_PASSES_SANITIZE_LEAK=true` but still finishes successfully with the leak checker enabled, then this indicates that the test is leak free and thus missing the annotation. It is somewhat slow to execute though because it runs all of our test suites with the leak sanitizer enabled. It is also pointless in most cases, because the only test suites that need to be checked are those which _aren't_ yet marked with `TEST_PASSES_SANITIZE_LEAK=true`. Introduce a new value "check-failing". When set, we behave the same as if "check" was passed, except that we only check those tests which do not have `TEST_PASSES_SANITIZE_LEAK=true` set. This is significantly faster than running all test suites but still fulfills the usecase of finding newly-leak-free test suites. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>