twister: Fix bug re-running all tests when using test-only

When using both test-only and retry-failed flags, all tests would be
added to the test queue regardless if they failed or not. This commit
updates the logic to process test-only runs similar as other runs,
by adding tests to the queue with the 'run' operation directly.

Signed-off-by: Andreas Vibeto <andreas.vibeto@nordicsemi.no>