Some tests can fail randomly (WriteDescendingRollingFile, WriteAscendingStableBaseRollingFile)

[VitaliyMF]

Reported here: #74 (comment)

I also noticed that sometimes tests related to this commit can fail (in my dev env, I often see WriteDescendingRollingFile failed).
It seems the problem is not in the FileLoggerProvider logic, and caused by a test that relies on System.Threading.Thread.Sleep(20).

[VitaliyMF]

This happens because tests too fast and file modified date is the same which breaks the logic that determines 'last' file.
Added more delays to fix this issue with tests.