Return bad error code when tests fail

src/Layout-test-utils.h

@@ -5,7 +5,7 @@
 #include <string.h>
 
 void test(const char *name, css_node_t *style, css_node_t *expected_layout);
-void tests_finished(void);
+int tests_finished(void);
 css_dim_t measure(void *context, float width);
 void init_css_node_children(css_node_t *node, int children_count);
 css_node_t *new_test_css_node(void);