Fix flaky StandbyModeE2E_LinuxContainer test

In v3.x, I've commented out the line that checks `dispatchDuration` in this test. Sometimes (and all the time lately in devops), this value comes back as 0 and the test fails. I added some explicit slowdowns to test so I think everything is working as expected, but we should look at this.

It looks like @divyagandhii / @mathewc / @maiqbal11 have touched this file in the past. Maybe we just need to make sure this value is there, and 0 is okay?

https://github.com/Azure/azure-functions-host/blob/dev/test/WebJobs.Script.Tests.Integration/Host/StandbyManager/StandbyManagerE2ETests_Linux.cs#L130

Start with test/WebJobs.Script.Tests.Integration/Host/StandbyManager/StandbyManagerE2ETests_Linux.cs around line 130, where the dispatchDuration check was commented out. Run StandbyModeE2E_LinuxContainer in a Linux container, especially in the devops environment, and compare runs where dispatchDuration is zero. Done means the test reliably reflects the expected behavior without masking a valid failure.

Written by the indexing model from the issue text.