Unable to recover in scheduler state machine after skipping tests.

Sometimes a test is invalid. Such as `test_downgrade_charm`, if there is no feasible charm to downgrade to.

If the test is skipped, we need some sort of recovery mechanism.

Start by locating the scheduler state machine and the handling of skipped tests. Reproduce the case described by test_downgrade_charm, where no feasible downgrade exists, and trace what state it remains in afterward. Done means a skipped invalid test has a defined recovery path and the scheduler continues correctly.

Written by the indexing model from the issue text.