Add dozens of new integration tests to the GDScript test suite

This also ignores `.out` files in the file format static checks.

modules/gdscript/tests/scripts/parser/warnings/unused_argument.gd

@@ -0,0 +1,12 @@
+# This should emit a warning since the unused argument is not prefixed with an underscore.
+func function_with_unused_argument(p_arg1, p_arg2):
+	print(p_arg1)
+
+
+# This shouldn't emit a warning since the unused argument is prefixed with an underscore.
+func function_with_ignored_unused_argument(p_arg1, _p_arg2):
+	print(p_arg1)
+
+
+func test():
+	pass