Improved handling of errors within test framework

Improvements

• Godot exits with error code 1 if test scene runner tries and fails to run
• SceneScout detects and handles (more) unrecoverable errors
• SceneScout is used when running individual test scenes (without test scene runner)
• SceneScout tries to work around wrong number of test runners within test scenes (either 0, or >1)

Fixes

• Minor .gitlab-ci.yml cleanup
• Add missing new/renamed resources to export config

Tests

• Test scene without any test runner