grep: add tests for --threads=N and grep.threads

Add tests for --threads=N being supplied on the command-line, or when grep.threads=N being supplied in the configuration. When the threading support was made run-time configurable in commit 89f09dd34e ("grep: add --threads=<num> option and grep.threads configuration", 2015-12-15) no tests were added for it. In developing a change to the grep code I was able to make '--threads=1 <pat>` segfault, while the test suite still passed. This change fixes that blind spot in the tests. In addition to asserting that asking for N threads shouldn't segfault, test that the grep output given any N is the same. The choice to test only 1..10 as opposed to 1..8 or 1..16 or whatever is arbitrary. Testing 1..1024 works locally for me (but gets noticeably slower as more threads are spawned). Given the structure of the code there's no reason to test an arbitrary number of threads, only 0, 1 and >=2 are special modes of operation. A later patch introduces a PTHREADS test prerequisite which is true under NO_PTHREADS=UnfortunatelyYes, but even under NO_PTHREADS it's fine to test --threads=N, we'll just ignore it and not use threading. So these tests also make sense under that mode to assert that --threads=N without pthreads still returns expected results. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>