Rename Project Gutenberg data files in test assets folder

Some of the files in https://github.com/dotnet/performance/tree/6e457b3e4145eeb7acd966cd166dc5ca3fb7381e/src/benchmarks/micro/libraries/Common are provided by Project Gutenberg and are required to maintain their original filenames per the license terms. We should rename these files to comply with their license terms. We should also update [_Perf.TextBase.cs_](https://github.com/dotnet/performance/blob/6e457b3e4145eeb7acd966cd166dc5ca3fb7381e/src/benchmarks/micro/libraries/Common/Perf.TextBase.cs) and related filed to perform friendly name mapping back to the actual filename mapping.

Required renames:

* __Chinese.txt__ -\> 25249-0.txt
* __Cyrillic.txt__ -\> 30774-0.txt
* __EnglishAllAscii.txt__ -\> 11.txt
* __EnglishMostlyAscii.txt__ -\> 11-0.txt
* __Greek.txt__ -\> 39251-0.txt