test-lib: Don't write test-results when HARNESS_ACTIVE

test-lib: Don't write test-results when HARNESS_ACTIVE

TAP harnesses don't need to read test-results/*, since they keep track
of the number of passing/failing tests internally. Skip the generation
of these files when HARNESS_ACTIVE is set.

It's now possible to run the Git test suite without writing anything
to the t/ directory at all if you use a TAP harness and the --root
switch:

    cd t
    sudo mount -t tmpfs none /tmp/memory -o size=300m
    prove -j9 ./t[0-9]*.sh :: --root=/tmp/memory

The I/O that the ~500 test-results/* files contributed was very
minimal, but I thought this was worth mentioning.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>