test(MockFileStream): use Read return value to satisfy CA2022

The shared-contents regression test ignored the byte count returned by
Stream.Read, which the analyzer flags as CA2022 (inexact read) and the
build treats as an error. Capture the count and assert it equals the
requested length.

Author: vbreuss

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs

@@ -33,7 +33,8 @@ public async Task MockFileStream_SharedReadWrite_SecondHandleSeesWritesFromFirst
 
                 file2.Position = 0;
                 file2.Flush();
-                file2.Read(buffer, 0, buffer.Length);
+                var bytesRead = file2.Read(buffer, 0, buffer.Length);
+                await That(bytesRead).IsEqualTo(buffer.Length);
                 results.Add(BitConverter.ToInt32(buffer, 0));
             }
         }