tests: add some script lint checks

There are some common but minor errors we tend to make in writing test scripts: 1. Scripts are left non-executable. This is not usually noticed immediately because "make test" does not need the bit, but it is a matter of git policy to make them executable (and is a slight convenience when running individual scripts). 2. Two scripts are allocated the same number. Usually this happens on separate branches, and the problem only comes about during a merge. But since there is no textual conflict, the merger would have to be very observant to notice. This is also a minor error, but can make GIT_SKIP_TESTS ambiguous. This patch introduces a "test-lint" target which checks both. It is not invoked by default. You can invoke it as "make test-lint", or you can make it a prerequisite of running the tests by specifying "TEST_LINT = test-lint" in your config.mak or on the command line. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>