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/GdUnitAssert.gd

@@ -0,0 +1,47 @@
+## Base interface of all GdUnit asserts
+@abstract class_name GdUnitAssert
+extends RefCounted
+
+
+## Verifies that the current value is null.
+@abstract func is_null() -> GdUnitAssert
+
+
+## Verifies that the current value is not null.
+@abstract func is_not_null() -> GdUnitAssert
+
+
+## Verifies that the current value is equal to expected one.
+@abstract func is_equal(expected: Variant) -> GdUnitAssert
+
+
+## Verifies that the current value is not equal to expected one.
+@abstract func is_not_equal(expected: Variant) -> GdUnitAssert
+
+
+## Overrides the default failure message by given custom message.[br]
+## This function allows you to replace the automatically generated failure message with a more specific
+## or user-friendly message that better describes the test failure context.[br]
+## Usage:
+##     [codeblock]
+##		# Override with custom context-specific message
+##		func test_player_inventory():
+##		    assert_that(player.get_item_count("sword"))\
+##		        .override_failure_message("Player should have exactly one sword")\
+##		        .is_equal(1)
+##     [/codeblock]
+@abstract func override_failure_message(message: String) -> GdUnitAssert
+
+
+## Appends a custom message to the failure message.[br]
+## This can be used to add additional information to the generated failure message
+## while keeping the original assertion details for better debugging context.[br]
+## Usage:
+##     [codeblock]
+##		# Add context to existing failure message
+##		func test_player_health():
+##		    assert_that(player.health)\
+##		        .append_failure_message("Player was damaged by: %s" % last_damage_source)\
+##		        .is_greater(0)
+##     [/codeblock]
+@abstract func append_failure_message(message: String) -> GdUnitAssert