Finish subexpr invariant test

tests/desugar_tests.c

@@ -146,6 +146,23 @@ static void subexpr_a_is_unchanged(void)
 	const bool success = desugar_regex(&t);
 	ASSERT_TRUE(success);
 
+	ASSERT_EQ(1, t.count);
+	ASSERT_NOT_NULL(t.contents);
+	ASSERT_EQ(1, t.contents[0].count);
+	ASSERT_NOT_NULL(t.contents[0].contents);
+	ASSERT_EQ(PARSE_QUANTIFIER_NONE, t.contents[0].contents[0].quantifier);
+	ASSERT_EQ(PARSE_TERM_SUBEXPR, t.contents[0].contents[0].type);
+
+	const parse_tree_t *inner = &t.contents[0].contents[0].subexpr;
+	ASSERT_EQ(1, inner->count);
+	ASSERT_NOT_NULL(inner->contents);
+	ASSERT_EQ(1, inner->contents[0].count);
+	ASSERT_NOT_NULL(inner->contents[0].contents);
+	ASSERT_EQ(
+	    PARSE_QUANTIFIER_NONE, inner->contents[0].contents[0].quantifier);
+	ASSERT_EQ(PARSE_TERM_LITERAL, inner->contents[0].contents[0].type);
+	ASSERT_EQ('a', inner->contents[0].contents[0].literal);
+
 	parse_free_tree_children(&t);
 }