test-lib: invert return value of check_test_results_san_file_empty

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>