Use [RetryFact] for policy unit tests

[bitfaster]

Absorb transient failures similar to:

[xUnit.net 00:00:01.37] --- End of stack trace from previous location --- 
Failed BitFaster.Caching.UnitTests.Lru.DiscretePolicyTests.TouchUpdatesTicksCount [484 ms] 
Error Message: 
Expected item.TickCount to be greater than 853681L, but found 853681L.

[coveralls]

coverage: 99.122% (+0.05%) from 99.072%
when pulling 39f809c on users/alexpeck/teststab
into d365e6d on main.