cocci: add a "coccicheck-test" target and test *.cocci rules

Add a "coccicheck-test" target to test our *.cocci rules, and as a demonstration add tests for the rules added in 39ea59a2570 (remove unnecessary NULL check before free(3), 2016-10-08) and 1b83d1251ed (coccinelle: add a rule to make "expression" code use FREE_AND_NULL(), 2017-06-15). I considered making use of the "spatch --test" option, and the choice of a "tests" over a "t" directory is to make these tests compatible with such a future change. Unfortunately "spatch --test" doesn't return meaningful exit codes, AFAICT you need to "grep" its output to see if the *.res is what you expect. There's "--test-okfailed", but I didn't find a way to sensibly integrate those (it relies on some in-between status files, but doesn't help with the status codes). Instead let's use a "--sp-file" pattern similar to the main "coccicheck" rule, with the difference that we use and compare the two *.res files with cmp(1). The --very-quiet and --no-show-diff options ensure that we don't need to pipe stdout and stderr somewhere. Unlike the "%.cocci.patch" rule we're not using the diff. The "cmp || git diff" is optimistically giving us better output on failure, but even if we only have POSIX cmp and no system git installed we'll still fail with the "cmp", just with an error message that isn't as friendly. The "2>/dev/null" is in case we don't have a "git" installed. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>