3.x: Fix recent groupBy tests sometimes failing with MBE (#6994)

Author: akarnokd

src/test/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableGroupByTest.java

@@ -2719,10 +2719,9 @@ public void issue6974Part2Case1ObserveOn() {
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
             , flatMapMaxConcurrency)
-        .test()
+        .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
-        .assertNoErrors()
-        .assertComplete();
+        .assertTerminated(); // MBE is possible if the async group closing is slow
     }
 
     @Test
@@ -2746,10 +2745,9 @@ public void issue6974Part2Case1ObserveOnHide() {
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
             , flatMapMaxConcurrency)
-        .test()
+        .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
-        .assertNoErrors()
-        .assertComplete();
+        .assertTerminated(); // MBE is possible if the async group closing is slow
     }
 
     @Test
@@ -2834,10 +2832,9 @@ public void issue6974Part2Case1ObserveOnConditional() {
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
             , flatMapMaxConcurrency)
-        .test()
+        .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
-        .assertNoErrors()
-        .assertComplete();
+        .assertTerminated(); // MBE is possible if the async group closing is slow
     }
 
     @Test
@@ -2862,10 +2859,9 @@ public void issue6974Part2Case1ObserveOnConditionalHide() {
                      // .take(10)
                      .take(10, TimeUnit.MILLISECONDS)
             , flatMapMaxConcurrency)
-        .test()
+        .subscribeWith(new TestSubscriberEx<>())
         .awaitDone(5, TimeUnit.SECONDS)
-        .assertNoErrors()
-        .assertComplete();
+        .assertTerminated(); // MBE is possible if the async group closing is slow
     }
 
     /*