test-lib: check for leak logs after every test

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>