Validate GOP test output with screenshots

[HadrienG2]

As discussed in #1, it would be nice to add GOP test output validation to CI. This PR does so by using the "screendump" functionality of QEMU's monitor.

Implementing this had some side-effects:

• Serial ports now implement fmt::Write for easily sending UTF-8 strings (removed)
• The previous "qemu-f4-exit" feature was turned into a more general "qemu" feature which enables all the features that are specific to our QEMU-based test environment.
• The python script can now chat with the QEMU monitor's JSON interface, which we could use later on for more advanced debugging features like core dumps.
• I removed all temporizations from automated tests, so they will run a few seconds faster :)

[GabrielMajeri]

LGTM 👍