Replace the broken test suite with working GdUnit4 tests

The previous tests never compiled: they used GUT's assert functions,
Godot 3 APIs and reassigned autoload functions, so no test ran.

- 90 tests: unit tests for the playlist, songs, player scoring, time
  formatting, search and the server client, plus integration tests
  that run MainWindow and SyncWindow against a fake server
- Keybind tests check that Alt+<number> and Ctrl+Alt+<number> keep
  following the player rows while players are added and removed
- run_tests.sh finds Godot and runs the suite headless (~10 seconds)
- CI runs the tests before the build job in gitea-build.yaml; the
  separate test.yml used a GitHub action that does not exist
- GdUnit4 is committed instead of ignored, since the project enables
  it, and .gitea/ is no longer ignored
- Exported builds leave out tests/ and addons/gdUnit4/

addons/gdUnit4/src/GdUnitObjectAssert.gd

@@ -0,0 +1,61 @@
+## An Assertion Tool to verify Object values
+@abstract class_name GdUnitObjectAssert
+extends GdUnitAssert
+
+
+## Verifies that the current value is null.
+@abstract func is_null() -> GdUnitObjectAssert
+
+
+## Verifies that the current value is not null.
+@abstract func is_not_null() -> GdUnitObjectAssert
+
+
+## Verifies that the current value is equal to the given one.
+@abstract func is_equal(expected: Variant) -> GdUnitObjectAssert
+
+
+## Verifies that the current value is not equal to expected one.
+@abstract func is_not_equal(expected: Variant) -> GdUnitObjectAssert
+
+
+## Overrides the default failure message by given custom message.
+@abstract func override_failure_message(message: String) -> GdUnitObjectAssert
+
+
+## Appends a custom message to the failure message.
+@abstract func append_failure_message(message: String) -> GdUnitObjectAssert
+
+
+## Verifies that the current object is the same as the given one.
+@abstract func is_same(expected: Variant) -> GdUnitObjectAssert
+
+
+## Verifies that the current object is not the same as the given one.
+@abstract func is_not_same(expected: Variant) -> GdUnitObjectAssert
+
+
+## Verifies that the current object is an instance of the given type.
+@abstract func is_instanceof(type: Variant) -> GdUnitObjectAssert
+
+
+## Verifies the current object is a valid instance (e.g. has not been freed from memory).[br]
+## [codeblock]
+## var node := Node.new()
+## assert_object(node).is_valid()  # passes — node is alive
+## node.free()
+## assert_object(node).is_valid()  # fails  — node has been freed
+## [/codeblock]
+@abstract func is_valid() -> GdUnitObjectAssert
+
+
+## Verifies that the current object is not an instance of the given type.
+@abstract func is_not_instanceof(type: Variant) -> GdUnitObjectAssert
+
+
+## Checks whether the current object inherits from the specified type.
+@abstract func is_inheriting(type: Variant) -> GdUnitObjectAssert
+
+
+## Checks whether the current object does NOT inherit from the specified type.
+@abstract func is_not_inheriting(type: Variant) -> GdUnitObjectAssert