Update test suite to avoid unhandled promise rejections

Author: clue

tests/CoroutineTest.php

@@ -193,6 +193,8 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesForPromiseReject
             });
         });
 
+        $promise->then(null, $this->expectCallableOnce()); // avoid reporting unhandled rejection
+
         unset($promise);
 
         $this->assertEquals(0, gc_collect_cycles());
@@ -232,6 +234,8 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorThr
             yield; // @phpstan-ignore-line
         });
 
+        $promise->then(null, $this->expectCallableOnce()); // avoid reporting unhandled rejection
+
         unset($promise);
 
         $this->assertEquals(0, gc_collect_cycles());
@@ -249,6 +253,8 @@ public function testCoroutineShouldNotCreateAnyGarbageReferencesWhenGeneratorYie
             yield 42;
         });
 
+        $promise->then(null, $this->expectCallableOnce()); // avoid reporting unhandled rejection
+
         unset($promise);
 
         $this->assertEquals(0, gc_collect_cycles());