build: Distinguish SKIP and FAIL on Win32

The `check` target in test/Makefile.win32 assumed that any non-0 exit
code from the tests was an error, but the testsuite is currently using
77 as a SKIP exit code (based on the convention used in autotools).

Fixes fence-image-self-test and cover-test (now reported as SKIP).

Signed-off-by: Andrea Canciani <ranma42@gmail.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>