[AppDomain] System.MissingMethodException because wrong assemblies are loaded into domain

Xamarin Studio+NUnit is using AppDomains to load .dlls and we always want to use our nunit.core.dll, nunit.utils.dll...
https://github.com/mono/monodevelop/blob/93c3aab/main/src/addins/MonoDevelop.UnitTesting.NUnit/NUnitRunner/NUnitTestRunner.cs#L53-L67

See attached project for reproduction of crash below. Same code works fine on .Net(it loads .dlls as expected from bin/Debug folder and not as Mono does from bin/copy folder.

```
Unhandled Exception:
System.MissingMethodException: Method 'NUnit.Core.TestSuiteBuilder.Build' not found.


Server stack trace:

  at (wrapper remoting-invoke-with-check) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at (wrapper xdomain-dispatch) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (object,byte[]&,byte[]&,string,string[])

Exception rethrown at [0]:

  at (wrapper xdomain-invoke) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at (wrapper remoting-invoke-with-check) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at MonoDevelop.UnitTesting.NUnit.External.RemoteNUnitTestRunner.GetTestInfo (System.String path) [0x00009] in <1ca860bd7d9040a0affd76b5bd2801c6>:0 
  at NUnitRunner.Application.Main (System.String[] args) [0x0005f] in <1ca860bd7d9040a0affd76b5bd2801c6>:0 

[ERROR] FATAL UNHANDLED EXCEPTION: System.MissingMethodException: Method 'NUnit.Core.TestSuiteBuilder.Build' not found.


Server stack trace:

  at (wrapper remoting-invoke-with-check) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at (wrapper xdomain-dispatch) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (object,byte[]&,byte[]&,string,string[])

Exception rethrown at [0]:

  at (wrapper xdomain-invoke) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at (wrapper remoting-invoke-with-check) MonoDevelop.UnitTesting.NUnit.External.NUnitTestRunner:GetTestInfo (string,string[])
  at MonoDevelop.UnitTesting.NUnit.External.RemoteNUnitTestRunner.GetTestInfo (System.String path) [0x00009] in <1ca860bd7d9040a0affd76b5bd2801c6>:0 
  at NUnitRunner.Application.Main (System.String[] args) [0x0005f] in <1ca860bd7d9040a0affd76b5bd2801c6>:0 

```


Reference: https://bugzilla.xamarin.com/show_bug.cgi?id=49308