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/multiline_if.gd

@@ -0,0 +1,14 @@
+func test():
+	# Line breaks are allowed within parentheses.
+	if (
+		1 == 1
+		and 2 == 2 and
+		3 == 3
+	):
+		pass
+
+	# Alternatively, backslashes can be used.
+	if 1 == 1 \
+		and 2 == 2 and \
+		3 == 3:
+		pass