Test that have been skipped because of a failed dependency cannot be re-run afterwards

[Pierre Equoy]

With Plainbox 0.21, we now have the possibility to re-run failed tests at the end of a session (before submitting it).

There is a problem with tests skipped because of a failed dependency.

Example:

1. Run a test plan that includes the three following tests:
    - usb/insert
    - usb/storage-automated
    - usb/remove
2. During `usb/insert`, fail to insert a USB stick within the given time (30 seconds).
→ the test will be marked as "failed", and `usb/storage-automated` and `usb/remove` will be marked as "skipped" because they depend on `usb/insert`.
3. Finish running the session
→ A re-run window shows up, but only `usb/insert` is proposed to be re-run (not `usb/storage-automated` nor `usb/remove`).

It would be better if tests that depend on the failed test (and therefore were skipped) are proposed to be re-run as well.

Other examples of such tests include:
    - mediacard/mmc-insert
    - mediacard/mmc-storage
    - mediacard/mmc-remove
    - mediacard/sd-insert
    - mediacard/sd-storage
    - mediacard/sd-remove
    - mediacard/sdhc-insert
    - mediacard/sdhc-storage
    - mediacard/sdhc-remove
    - usb3/insert
    - usb3/storage-automated
    - usb3/remove
    - ...