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/GdUnitIntAssert.gd

@@ -0,0 +1,79 @@
+## An Assertion Tool to verify integer values
+@abstract class_name GdUnitIntAssert
+extends GdUnitAssert
+
+
+## Verifies that the current value is null.
+@abstract func is_null() -> GdUnitIntAssert
+
+
+## Verifies that the current value is not null.
+@abstract func is_not_null() -> GdUnitIntAssert
+
+
+## Verifies that the current value is equal to the given one.
+@abstract func is_equal(expected: Variant) -> GdUnitIntAssert
+
+
+## Verifies that the current value is not equal to expected one.
+@abstract func is_not_equal(expected: Variant) -> GdUnitIntAssert
+
+
+## Overrides the default failure message by given custom message.
+@abstract func override_failure_message(message: String) -> GdUnitIntAssert
+
+
+## Appends a custom message to the failure message.
+@abstract func append_failure_message(message: String) -> GdUnitIntAssert
+
+
+## Verifies that the current value is less than the given one.
+@abstract func is_less(expected: int) -> GdUnitIntAssert
+
+
+## Verifies that the current value is less than or equal the given one.
+@abstract func is_less_equal(expected: int) -> GdUnitIntAssert
+
+
+## Verifies that the current value is greater than the given one.
+@abstract func is_greater(expected: int) -> GdUnitIntAssert
+
+
+## Verifies that the current value is greater than or equal the given one.
+@abstract func is_greater_equal(expected: int) -> GdUnitIntAssert
+
+
+## Verifies that the current value is even.
+@abstract func is_even() -> GdUnitIntAssert
+
+
+## Verifies that the current value is odd.
+@abstract func is_odd() -> GdUnitIntAssert
+
+
+## Verifies that the current value is negative.
+@abstract func is_negative() -> GdUnitIntAssert
+
+
+## Verifies that the current value is not negative.
+@abstract func is_not_negative() -> GdUnitIntAssert
+
+
+## Verifies that the current value is equal to zero.
+@abstract func is_zero() -> GdUnitIntAssert
+
+
+## Verifies that the current value is not equal to zero.
+@abstract func is_not_zero() -> GdUnitIntAssert
+
+
+## Verifies that the current value is in the given set of values.
+@abstract func is_in(expected: Array) -> GdUnitIntAssert
+
+
+## Verifies that the current value is not in the given set of values.
+@abstract func is_not_in(expected: Array) -> GdUnitIntAssert
+
+
+## Verifies that the current value is between the given boundaries (inclusive).
+@abstract func is_between(from: int, to: int) -> GdUnitIntAssert