Rewrite the MemoryMappedFiles tests

The old System.IO.MemoryMappedFiles tests had decent coverage, but were lacking in a variety of ways:
- They were written in a legacy style that didn't take advantage of xunit capabilities, that made them difficult to augment, and that made them difficult to understand
- They were spread out across several projects in a seemingly haphazard manner.
- They left behind lots of temporary files
- They caused random test failures due to conflicting map names and file names.
- Their code coverage could have been a bit better, and they were missing coverage of some corner cases

This commit completely rewrites the tests, covering all of the old scenarios tested and more while removing 2/3rds of the code, increasing code coverage, avoiding race conditions on map and file names, cleaning up temporary files used, switching to using xunit features, and hopefully making the tests more understandable and augmentable in the future.

Author: stephentoub

src/System.IO.MemoryMappedFiles/System.IO.MemoryMappedFiles.sln

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