test-lib: use individual lsan dir for --stress runs

test-lib: use individual lsan dir for --stress runs

When storing output in test-results/, we usually give each numbered run
in a --stress set its own output file. But we don't do that for storing
LSan logs, so something like:

  ./t0003-attributes.sh --stress

will have many scripts simultaneously creating, writing to, and deleting
the test-results/t0003-attributes.leak directory. This can cause logs
from one run to be attributed to another, spurious failures when
creation and deletion race, and so on.

This has always been broken, but nobody noticed because it's rare to do
a --stress run with LSan (since the point is for the code to run quickly
many times in order to hit races). But if you're trying to find a race
in the leak sanitizing code, it makes sense to use these together.

We can fix it by using $TEST_RESULTS_BASE, which already incorporates
the stress job suffix.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>