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/GdUnitDictionaryAssert.gd

@@ -0,0 +1,79 @@
+## An Assertion Tool to verify dictionary
+@abstract class_name GdUnitDictionaryAssert
+extends GdUnitAssert
+
+
+## Verifies that the current value is null.
+@abstract func is_null() -> GdUnitDictionaryAssert
+
+
+## Verifies that the current value is not null.
+@abstract func is_not_null() -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is equal to the given one, ignoring order.
+@abstract func is_equal(expected: Variant) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is not equal to the given one, ignoring order.
+@abstract func is_not_equal(expected: Variant) -> GdUnitDictionaryAssert
+
+
+## Overrides the default failure message by given custom message.
+@abstract func override_failure_message(message: String) -> GdUnitDictionaryAssert
+
+
+## Appends a custom message to the failure message.
+@abstract func append_failure_message(message: String) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is empty, it has a size of 0.
+@abstract func is_empty() -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is not empty, it has a size of minimum 1.
+@abstract func is_not_empty() -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is the same. [br]
+## Compares the current by object reference equals
+@abstract func is_same(expected: Variant) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary is NOT the same. [br]
+## Compares the current by object reference equals
+@abstract func is_not_same(expected: Variant) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary has a size of given value.
+@abstract func has_size(expected: int) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary contains the given key(s).[br]
+## The keys are compared by deep parameter comparision, for object reference compare you have to use [method contains_same_keys]
+@abstract func contains_keys(...expected: Array) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary contains the given key and value.[br]
+## The key and value are compared by deep parameter comparision, for object reference compare you have to use [method contains_same_key_value]
+@abstract func contains_key_value(key: Variant, value: Variant) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary not contains the given key(s).[br]
+## The keys are compared by deep parameter comparision, for object reference compare you have to use [method not_contains_same_keys]
+@abstract func not_contains_keys(...expected: Array) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary contains the given key(s).[br]
+## The keys are compared by object reference, for deep parameter comparision use [method contains_keys]
+@abstract func contains_same_keys(expected: Array) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary contains the given key and value.[br]
+## The key and value are compared by object reference, for deep parameter comparision use [method contains_key_value]
+@abstract func contains_same_key_value(key: Variant, value: Variant) -> GdUnitDictionaryAssert
+
+
+## Verifies that the current dictionary not contains the given key(s).
+## The keys are compared by object reference, for deep parameter comparision use [method not_contains_keys]
+@abstract func not_contains_same_keys(...expected: Array) -> GdUnitDictionaryAssert