Expose underlying unix file descriptor in SafeMemoryMappedFileHandle (#53538)

Expose underlying unix file descriptor in SafeMemoryMappedFileHandle (#53538)

* expose unix file descriptor safe memory map

* store handle after DangerousAddRef

* add test verifying handle matches filestream handle

* Update src/libraries/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.Unix.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.Unix.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.Unix.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.Unix.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.IO.MemoryMappedFiles/tests/MemoryMappedFile.CreateFromFile.Tests.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Dispose MemoryMappedFile in MapHandleMatchesFileStreamHandle test

* Update src/libraries/System.IO.MemoryMappedFiles/src/Microsoft/Win32/SafeMemoryMappedFileHandle.Unix.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update MemoryMappedFile.CreateFromFile.Tests.cs

* Move mmf in test to using block

* Fix test

Co-authored-by: Danny Friar <danny@Dannys-MacBook-Pro.local>
Co-authored-by: Stephen Toub <stoub@microsoft.com>