Increase timeout for background scheduler test

[bitfaster]

Since switching to the default scheduler, this test has failed to stop twice. Async/await and time are not stable in xunit. As a first step to avoid intermittent failures, try to increase the timeout.

System.Exception : Failed to stop
   at BitFaster.Caching.UnitTests.Scheduler.BackgroundSchedulerTests.WhenDisposedRunsToCompletion() in D:\a\BitFaster.Caching\BitFaster.Caching\BitFaster.Caching.UnitTests\Scheduler\BackgroundSchedulerTests.cs:line 92
--- End of stack trace from previous location where exception was thrown ---

[coveralls]

coverage: 98.292% (-0.4%) from 98.667%
when pulling 06a3c6a on users/alexpeck/schedstab
into 694d043 on main.