test_etcp_router_unit: fix FAIL macro — bare return in non-void function

tests/test_etcp_router_unit.c

@@ -111,7 +111,7 @@ static int test_total = 0, test_passed = 0, test_failed = 0;
 } while(0)
 
 #define PASS() do { puts("PASS"); test_passed++; } while(0)
-#define FAIL(msg) do { printf("FAIL: %s\n", msg); test_failed++; return; } while(0)
+#define FAIL(msg) do { printf("FAIL: %s\n", msg); test_failed++; return -1; } while(0)
 
 // ======================== Tests ========================