failure detection fixes & clean qemu shutdown

[gratux]

During this semester of OS, we had the pleasure of using your tool.
We noticed some issues with test status detection:

• timeouts were displayed in the log, but not counted towards the overall status. As such, when all tests either pass or timeout, tortillas yields success as the exit code.
• if someone disabled vital kernel logs (i.e. SYSCALL for exit code detection), tortillas didn't detect any failures and marked tests as success. In our opinion, it should instead mark tests expecting a specific exit code as failed, since no exit code was found at all.

Additionally, we added a clean QEMU shutdown, so changes to the file system are committed to disk. One may then inspect the image and its contents afterwards.

[PaideiaDilemma]

Thanks for making this PR!