tests: move gobject deps to a fixture

[Saviq]

Decided to drop the executable and package checks as these will be obvious from test failures anyway.

[Saviq]

program.ProgramError: qterminal closed with exit code -15

This seems to happen from time to time…

[Saviq]

@wmww can confirm this makes the test run fine in the lab. It complains it only collected 2 frames, though.

[bors[bot]]

Build succeeded:

• ProcessSnaps
• RunTests (3.10, self)