patman: test_util: Handle nonexistent tests while loading tests

It's possible to request a specific test to run when trying to run a
python tool's tests. If we request a nonexistent test, the unittest
loaders generate a fake test that reports this as an error. However, we
get these fake tests even when the test exists, because test_util can
load tests from multiple places one by one and the test we want only
exists in one.

The test_util helpers currently remove these fake tests when printing
test results, but that's more of a workaround than a proper solution.
Instead, don't even try to load the missing tests.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>