For example, valgrind can't handle some things, so we may need to
switch it off, but we don't want to do that for every test.

So, start a standard format for per-file restrictions, eg:

    Ccanlint: tests_pass_valgrind test/foo.c:FAIL

For the moment, only tests_pass_valgrind takes options, so it has to
change: we now have a helper which returns the array of options
applicable to a given file.