kunit: fix executor OOM error handling logic on non-UML

The existing logic happens to work fine on UML, but is not correct when
running on other arches.

1. We didn't initialize `int err`, and kunit_filter_suites() doesn't
   explicitly set it to 0 on success. So we had false "failures".
   Note: it doesn't happen on UML, causing this to get overlooked.
2. If we error out, we do not call kunit_handle_shutdown().
   This makes kunit.py timeout when using a non-UML arch, since the QEMU
   process doesn't ever exit.

Fixes: a02353f4 ("kunit: bail out of test filtering logic quicker if OOM")
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>