Integration tests are flaky

[kenzieschmoll]

They are currently disabled because they are slowing development.

This is from the integration_dart2js bot.

02:20 +12 -1: /home/runner/work/devtools/devtools/packages/devtools_app/test/integration_tests/server_test.dart: Server (VM Service) Server reuses DevTools instance if not connected to a VM [E]      
  Server did not return any known clients that are connected
  test/integration_tests/integration.dart 41:3  waitFor

@jacob314 I know you did some work here recently to reduce flakes and potential race conditions, but looks like the flakiness is back. I have re-run the bot probably 10 times today and it flaked each time. The PR I was running on (https://github.com/flutter/devtools/pull/2592) was all cosmetic UI changes, too, so it shouldn't have introduced new timing issues.