test-lib: check for leak logs after every test

If you are trying to find and fix leaks in a large test script, it can be overwhelming to see the leak logs for every test at once. The previous commit let you use "--immediate" to see the logs after the first failing test, but this isn't always the first leak. As discussed there, we may see leaks from previous tests that didn't happen to fail. To catch those, let's check for any logs that appeared after each test snippet is run, meaning that in a SANITIZE=leak build, any leak is an immediate failure of the test snippet. This check is mostly free in non-leak builds (just a "test -z"), and only a few extra processes in a leak build, so I don't think the overhead should matter (if it does, we could probably optimize for the common "no logs" case without even spending a process). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>