test: add assert() for rector errors

Author: kenjis

tests/system/CommonSingleServiceTest.php

@@ -36,6 +36,9 @@ public function testSingleServiceWithNoParamsSupplied(string $service): void
         $service1 = single_service($service);
         $service2 = single_service($service);
 
+        assert($service1 !== null);
+        assert($service1 !== null);
+
         $this->assertInstanceOf(get_class($service1), $service2);
         $this->assertNotSame($service1, $service2);
     }
@@ -64,6 +67,9 @@ public function testSingleServiceWithAtLeastOneParamSupplied(string $service): v
         $service1 = single_service($service, ...$params);
         $service2 = single_service($service, ...$params);
 
+        assert($service1 !== null);
+        assert($service1 !== null);
+
         $this->assertInstanceOf(get_class($service1), $service2);
         $this->assertNotSame($service1, $service2);
 
@@ -77,6 +83,9 @@ public function testSingleServiceWithAllParamsSupplied(): void
         $cache1 = single_service('cache', null, true);
         $cache2 = single_service('cache', null, true);
 
+        assert($cache1 !== null);
+        assert($cache2 !== null);
+
         // Assert that even passing true as last param this will
         // not create a shared instance.
         $this->assertInstanceOf(get_class($cache1), $cache2);