test-lib.sh: add --long-tests option

test-lib.sh: add --long-tests option

Add a --long-tests option to test-lib.sh, which enables tests to
selectively run more exhaustive (longer running, potentially
brute-force) tests.  Such exhaustive tests would only be useful if one
works on the specific module that is being tested -- for a general "cd
t/; make" to check whether everything is OK, such exhaustive tests
shouldn't be run by default since the longer it takes to run the
tests, the less often they are actually run.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>