service_manager_test.dart causing high number of bot flakes

[kenzieschmoll]

This is greatly slowing down development as we have to run the bots several times per CL to try to get the flake to stop happening. We should chase this down and make the service_manager_test more robust.

06:02 +353 ~3 -1: /home/runner/work/devtools/devtools/packages/devtools_app/test/service_manager_test.dart: ServiceConnectionManager toggle boolean service extension [E]                              
  TimeoutException after 0:02:00.000000: Test timed out after 2 minutes.
  dart:isolate  _RawReceivePortImpl._handleMessage
...
08:02 +353 ~3 -2: /home/runner/work/devtools/devtools/packages/devtools_app/test/service_manager_test.dart: ServiceConnectionManager toggle String service extension [E]                               
  TimeoutException after 0:02:00.000000: Test timed out after 2 minutes.
  dart:isolate  _RawReceivePortImpl._handleMessage
...
10:02 +353 ~3 -3: /home/runner/work/devtools/devtools/packages/devtools_app/test/service_manager_test.dart: ServiceConnectionManager toggle numeric service extension [E]                              
  TimeoutException after 0:02:00.000000: Test timed out after 2 minutes.
  dart:isolate  _RawReceivePortImpl._handleMessage