Multiple instances of Nunit runners, which use nunit-agent, can not be run in parallel.

[Armin Häberling]

An instance of an Nunit runner, which uses nunit-agent to run tests, is preventing any other parallel instance to execute tests using nunit-agent.
If the other instance uses /process=Separate, it fails with "No fixture found".
If it uses /process=Multiple, it fails with the following exception:

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei NUnit.Core.ProxyTestRunner.CountTestCases(ITestFilter filter)
   bei NUnit.Util.AggregatingTestRunner.CountTestCases(ITestFilter filter)
   bei NUnit.Util.AggregatingTestRunner.Run(EventListener listener, ITestFilter filter)
   bei NUnit.ConsoleRunner.ConsoleUi.Execute(ConsoleOptions options)
   bei NUnit.ConsoleRunner.Runner.Main(String[] args)

Steps to reproduce:
- for any test assembly start two instances of nunit-console setting /process=Separate or /process=Multiple

[Armin Häberling (arminha)]

Note that running test from .net 4.0 assemblies usually implies /process=Separate unless the Nunit run under .net 4.0.
To force Nunit to run under .net 4.0 set the environment variable COMPLUS_Version=v4.0.30319 before starting Nunit.