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/shadowed_constant.gd

@@ -0,0 +1,8 @@
+# See also `parser-errors/redefine-class-constant.gd`.
+const TEST = 25
+
+
+func test():
+	# Warning here. This is not an error because a new constant is created,
+	# rather than attempting to set the value of an existing constant.
+	const TEST = 50