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/features/operator_assign.gd

@@ -0,0 +1,8 @@
+func test():
+	var i = 0
+	i += 5
+	i -= 4
+	i *= 10
+	i %= 8
+	i /= 0.25
+	print(round(i))