Cache expiration tests are flacky

[hazzik]

I suspect that one second gap might be not enough.

So far these are the tests that often fail:

• NHibernate.Caches.Common.Tests.CacheFixture.TestDistributedClear
• NHibernate.Caches.Common.Tests.CacheFixture.TestObjectExpirationAfterUpdate
• NHibernate.Caches.Common.Tests.CacheFixture.TestSlidingExpiration
• NHibernate.Caches.StackExchangeRedis.Tests.DistributedLocalCacheStrategyFixture.TestLockUnlockManyAsync
• NHibernate.Caches.StackExchangeRedis.Tests.DistributedLocalCacheStrategyFixture.TestObjectExpiration("cache.default_expiration")
• NHibernate.Caches.StackExchangeRedis.Tests.DistributedLocalCacheStrategyFixture.TestDistributedClear
• NHibernate.Caches.StackExchangeRedis.Tests.DistributedLocalCacheStrategyFixture.TestObjectExpiration("expiration")

[fredericDelaporte]

One second gap is still already a lot, especially for tests running entirely locally, without actual network access involved.

I guess we could still try increasing it and run the test many time from the branch to see if it solves the issue.