[release/6.0] Make tests for FileLoggerProcessor system time independent (#42115)

* Fix conflicts

* Fixup

* Fixup

* Update src/Middleware/HttpLogging/test/FileLoggerProcessorTests.cs

Co-authored-by: Benedikt <benedikt.achatz@gmail.com>
Co-authored-by: Doug Bunting <6431421+dougbu@users.noreply.github.com>