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/assert_always_true.gd

@@ -0,0 +1,5 @@
+func test():
+	# These statements always evaluate to `true`, and therefore emit a warning.
+	assert(true)
+	assert(-1.234)
+	assert(2 + 3 == 5)