CI: Add problem matcher for Godot, fail tests if triggered

[Repiteo]

There's been quite a lot of additions to tests lately that absolutely explode the CI output with error messages. This attempts to be a twofold solution to the problem:

• Add dedicated problem-matcher for Godot errors, so GHA can track them
• Detect if any warnings/errors aren't properly suppressed & fail a --test operation if so

The former worked like a charm, and the latter mostly works, but there's still some discrepancies that need ironing out. Namely, figuring out where exactly errors are accumulating when outputs are supposed to be suppressed. This might come down to how there's a shocking amount of places where print functionality can be toggled, causing weird overlaps. But the core of the functionality is in place: CI will not pass if warning/error output is not properly suppressed!