Flaky tests due to Kestrel engine

[Kaliumhexacyanoferrat]

Since we introduced Kestrel we randomly have errors such as this one in the CI build:

  Failed TestContentNotInvalidated (Kestrel) [11 ms]
  Error Message:
   Test method GenHTTP.Testing.Acceptance.Modules.ServerCaching.ServerCacheTests.TestContentNotInvalidated threw exception: 
System.IO.IOException: The process cannot access the file '/tmp/tmpfzmlsq.tmp' because it is being used by another process.
  Stack Trace:
      at Microsoft.Win32.SafeHandles.SafeFileHandle.Init(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Int64& fileLength, UnixFileMode& filePermissions)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Boolean failForSymlink, Boolean& wasSymlink, Func`4 createOpenException)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, Int64 preallocationSize)
   at System.IO.File.Create(String path, Int32 bufferSize, FileOptions options)
   at GenHTTP.Testing.Acceptance.Utilities.FileUtil.WriteTextAsync(String path, String content) in /home/runner/work/GenHTTP/GenHTTP/Testing/Acceptance/Utilities/FileUtil.cs:line 18
   at GenHTTP.Testing.Acceptance.Modules.ServerCaching.ServerCacheTests.TestContentNotInvalidated(TestEngine engine) in /home/runner/work/GenHTTP/GenHTTP/Testing/Acceptance/Modules/ServerCaching/ServerCacheTests.cs:line 71
   at GenHTTP.Testing.Acceptance.Modules.ServerCaching.ServerCacheTests.TestContentNotInvalidated(TestEngine engine) in /home/runner/work/GenHTTP/GenHTTP/Testing/Acceptance/Modules/ServerCaching/ServerCacheTests.cs:line 76

[Kaliumhexacyanoferrat]

Probably a race condition where we write the last bytes (of the known content length) to the output stream, Kestrel flushes them, the test continues its execution and then the disposal of the file stream (and its handle) happens.