patch 9.0.1179: not all errors around inheritance are tested

Problem:    Not all errors around inheritance are tested.
Solution:   Add more tests.  Fix uncovered problems.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 9fe8507..cc7a3bd 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -49,6 +49,20 @@
 	    && (cctx->ctx_ufunc->uf_flags & (FC_OBJECT|FC_NEW)))
     {
 	int is_super = *name == 's';
+	if (is_super)
+	{
+	    if (name[5] != '.')
+	    {
+		emsg(_(e_super_must_be_followed_by_dot));
+		return FAIL;
+	    }
+	    if (cctx->ctx_ufunc->uf_class != NULL
+		    && cctx->ctx_ufunc->uf_class->class_extends == NULL)
+	    {
+		emsg(_(e_using_super_not_in_child_class));
+		return FAIL;
+	    }
+	}
 	if (lvar != NULL)
 	{
 	    CLEAR_POINTER(lvar);