Reduce flakiness in io.opentelemetry.javaagent.instrumentation.grpc.v1_6.GrpcTest.clientCallAfterServerCompleted()

Automated fix attempt based on Develocity flaky-test analysis.

Author: trask

instrumentation/grpc-1.6/testing/src/main/java/io/opentelemetry/instrumentation/grpc/v1_6/AbstractGrpcTest.java

@@ -1533,7 +1533,7 @@ public void sayHello(
     // for the two cases due to lack of context propagation in the library case, but that isn't what
     // we're testing here.
 
-    clientCallDone.await(10, SECONDS);
+    assertThat(clientCallDone.await(10, SECONDS)).isTrue();
 
     assertThat(error).hasValue(null);
   }