Enable parallel tests

Enable parallel tests

On multiprocessor machines, or with I/O heavy tests (that leave the
CPU waiting a lot), it makes sense to parallelize the tests.

However, care has to be taken that the different jobs use different
trash directories.

This commit does so, by creating the trash directories with a suffix
that is unique with regard to the test, as it is the test's base name.

Further, the trash directory is removed in the test itself if
everything went fine, so that the trash directories do not
pile up only to be removed at the very end.

If a test failed, the trash directory is not removed.  Chances are
that the exact error message is lost in the clutter, but you can still
see what test failed from the name of the trash directory, and repeat
the test (without -j).

If all was good, you will see the aggregated results.

Suggestions to simplify this commit came from Junio and René.

There still is an issue with tests that want to run a server process and
listen to a fixed port (http and svn) --- they cannot run in parallel but
this patch does not address this issue.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>