test-lib: invert return value of check_test_results_san_file_empty

We have a function to check whether LSan logged any leaks. It returns success for no leaks, and non-zero otherwise. This is the simplest thing for its callers, who want to say "if no leaks then return early". But because it's implemented as a shell pipeline, you end up with the awkward: ! find ... | xargs grep leaks | grep -v false-positives where the "!" is actually negating the final grep. Switch the return value (and name) to return success when there are leaks. This should make the code a little easier to read, and the negation in the callers still reads pretty naturally. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>