nunit.engine.tests are failing to unload on master branch

[ChrisMaddock]

The nunit.engine.tests are currently failing to unload incorrectly on the master branch. I haven't had the time yet to bisect and find out where this crept in - if anyone else has the time to take a look, please feel free!

NUnit.Engine.NUnitEngineUnloadException : Agent Process was terminated successfully after error.
  ----> NUnit.Engine.NUnitEngineUnloadException : Exception encountered unloading application domain
  ----> NUnit.Engine.NUnitEngineException : Exception encountered unloading application domain: Error while unloading appdomain. (Exception from HRESULT: 0x80131015)
Application domain name: domain-
Application domain BaseDirectory: D:\a\1\s\bin\Release\net35\
Error trying to read application domain details: Could not load file or assembly 'nunit.engine, Version=3.13.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb' or one of its dependencies. The system cannot find the file specified.

[ChrisMaddock]

@CharliePoole this seems to have been introduced in #937. Would you mind taking a look and seeing if you can figure it out?

I can't see anything obvious myself - I wonder if the new TcpTransport class isn't disposed correctly in some test setup somewhere...

[ChrisMaddock]

Never mind, I got there eventually! PR incoming. 👍