patch 8.1.0386: cannot test with non-default option value

Problem:    Cannot test with non-default option value.
Solution:   Add test_option_not_set().

src/option.c

@@ -12480,13 +12480,17 @@ option_was_set(char_u *name)
 /*
  * Reset the flag indicating option "name" was set.
  */
-    void
+    int
 reset_option_was_set(char_u *name)
 {
     int idx = findoption(name);
 
     if (idx >= 0)
+    {
 	options[idx].flags &= ~P_WAS_SET;
+	return OK;
+    }
+    return FAIL;
 }
 
 /*