Update `RarHeaderFactoryTests` and `GZipArchiveTests` to open the test
readers as `FileAccess.Read` and `FileShare.Read` to prevent issues with
multiple test from trying to open exclusive access to files.
Tests fail in Visual Studio because they try to reuse the same scratch
working space, and each test is responsible for resetting the space. To
simplify the test code:
1. Make `TestBase` `IDisposable` and have it create the scratch space
2. Remove `ResetScratch()` as it is now handled by the base class
3. Add a unique ID to each scrach space folder to prevent collisions
