virttest.utils_test: Find sub test files using the asset APIs

This fixes autotest/tp-qemu#14.

Search in the provider structure for subtests,
fixing a problem that came with the repository
split.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Lin Ma <lma@suse.com>
Merge pull request #1524 from autotest/github-1499

virttest.utils_test: Find sub test files using the asset APIs