Fix buffering bug

[adamsitnik]

private ReadSpan and WriteSpan methods of the BufferedFileStreamStrategy accept both a Span and an ArraySegment for reasons described here:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

Lines 203 to 206 in cf842f1

	// For async file stream strategies the call to Read(Span) is translated to Stream.Read(Span),
	// which rents an array from the pool, copies the data, and then calls Read(Array). This is expensive!
	// To avoid that (and code duplication), the Read(Array) method passes ArraySegment to this method
	// which allows for calling Strategy.Read(Array) instead of Strategy.Read(Span).

The bug that I've discovered while working on #50166 was that the Span was getting sliced, while ArraySegment was not. This was causing a serious bug (but the new strategy was not enabled yet so we are safe):

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

Lines 538 to 542 in cf842f1

	{
	source.Slice(0, numBytes).CopyTo(_buffer!.AsSpan(_writePos));
	_writePos += numBytes;
	source = source.Slice(numBytes);
	}

Read is free of this bug as the ArraySegment is properly sliced using n:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

Lines 258 to 260 in cf842f1

	int moreBytesRead = arraySegment.Array != null
	? _strategy.Read(arraySegment.Array, arraySegment.Offset + n, arraySegment.Count - n)
	: _strategy.Read(destination.Slice(n));

I've added a failing tests to the FileStream conformance tests:

[xUnit.net 00:00:08.68]     System.IO.Tests.BufferedAsyncFileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(mode: SyncArray) [FAIL]
  Failed System.IO.Tests.BufferedAsyncFileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(mode: SyncArray) [13 ms]
  Error Message:
   Assert.Equal() Failure
Expected: 51
Actual:   52
  Stack Trace:
     at System.IO.Tests.FileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(ReadWriteMode mode) in C:\Projects\runtime\src\libraries\System.IO.FileSystem\tests\FileStream\FileStreamConformanceTests.cs:line 128
--- End of stack trace from previous location ---
[xUnit.net 00:00:08.76]     System.IO.Tests.BufferedSyncFileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(mode: SyncArray) [FAIL]
  Failed System.IO.Tests.BufferedSyncFileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(mode: SyncArray) [3 ms]
  Error Message:
   Assert.Equal() Failure
Expected: 51
Actual:   52
  Stack Trace:
     at System.IO.Tests.FileStreamStandaloneConformanceTests.NoDataIsLostWhenWritingToFile(ReadWriteMode mode) in C:\Projects\runtime\src\libraries\System.IO.FileSystem\tests\FileStream\FileStreamConformanceTests.cs:line 128
--- End of stack trace from previous location ---

[carlossanlop]

LGTM, assuming all tests pass.

A Mono CI leg failed with an unrelated error, I re-ran it, hoping it will pass this time:

##[error]Artifact CoreCLRProduct___OSX_x64_release not found for build 1058275. Please ensure you have published artifacts in any previous phases of the current build.