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

This is a remake of PR #1499.

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>