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/GdUnitGodotErrorAssert.gd

@@ -0,0 +1,59 @@
+## An assertion tool to verify for Godot runtime errors like assert() and push notifications like push_error().
+@abstract class_name GdUnitGodotErrorAssert
+extends GdUnitAssert
+
+
+## Verifies that the current value is null.
+@abstract func is_null() -> GdUnitGodotErrorAssert
+
+
+## Verifies that the current value is not null.
+@abstract func is_not_null() -> GdUnitGodotErrorAssert
+
+
+## Verifies that the current value is equal to the given one.
+@abstract func is_equal(expected: Variant) -> GdUnitGodotErrorAssert
+
+
+## Verifies that the current value is not equal to expected one.
+@abstract func is_not_equal(expected: Variant) -> GdUnitGodotErrorAssert
+
+
+## Overrides the default failure message by given custom message.
+@abstract func override_failure_message(message: String) -> GdUnitGodotErrorAssert
+
+
+## Appends a custom message to the failure message.
+@abstract func append_failure_message(message: String) -> GdUnitGodotErrorAssert
+
+
+## Verifies if the executed code runs without any runtime errors
+## Usage:
+##     [codeblock]
+##		await assert_error(<callable>).is_success()
+##     [/codeblock]
+@abstract func is_success() -> GdUnitGodotErrorAssert
+
+
+## Verifies if the executed code runs into a runtime error
+## Usage:
+##     [codeblock]
+##		await assert_error(<callable>).is_runtime_error(<expected error message>)
+##     [/codeblock]
+@abstract func is_runtime_error(expected_error: Variant) -> GdUnitGodotErrorAssert
+
+
+## Verifies if the executed code has a push_warning() used
+## Usage:
+##     [codeblock]
+##		await assert_error(<callable>).is_push_warning(<expected push warning message>)
+##     [/codeblock]
+@abstract func is_push_warning(expected_warning: Variant) -> GdUnitGodotErrorAssert
+
+
+## Verifies if the executed code has a push_error() used
+## Usage:
+##     [codeblock]
+##		await assert_error(<callable>).is_push_error(<expected push error message>)
+##     [/codeblock]
+@abstract func is_push_error(expected_error: Variant) -> GdUnitGodotErrorAssert