GDScript: Fix test for read-only state of constants

modules/gdscript/tests/scripts/runtime/features/constants_are_read_only.gd

@@ -0,0 +1,10 @@
+const array: Array = [0]
+const dictionary := {1: 2}
+
+@warning_ignore("assert_always_true")
+func test():
+	assert(array.is_read_only() == true)
+	assert(str(array) == '[0]')
+	assert(dictionary.is_read_only() == true)
+	assert(str(dictionary) == '{ 1: 2 }')
+	print('ok')

modules/gdscript/tests/scripts/runtime/features/constants_are_read_only.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+ok