[gardening] Fix isolate_group_shared_callback_test so it allows for slow execution.

Accept the fact that on slow platforms(like android arm devices) it might take longer for callback to respond.

Fixes #60700
TEST=ci

Change-Id: I67e322b29aaec1cf926790b5eeba67a411a8eaec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427801
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: aam

tests/ffi/isolate_group_shared_callback_test.dart

@@ -96,7 +96,7 @@ Future<void> testNativeCallableHelloWorld() async {
   mutexCondvar.runLocked(() {
     while (!resultIsReady) {
       conditionVariable.wait(mutexCondvar, 10 * sleepForMs);
-      Expect.isTrue(resultIsReady);
+      print('.');
     }
   });
 
@@ -107,7 +107,7 @@ Future<void> testNativeCallableHelloWorld() async {
   mutexCondvar.runLocked(() {
     while (!resultIsReady) {
       conditionVariable.wait(mutexCondvar, 10 * sleepForMs);
-      Expect.isTrue(resultIsReady);
+      print('.');
     }
   });
   Expect.equals(42 + (1001 * 123) * 2, result);