When such an assembly is being tested in NUnit, with Shadow Copying enabled (the default setting), can generate a file path that exceeds the maximum length of file path name of Windows.
When this happens, it can generate an exception saying that it cannot create an instance of MCR. Check the Text output tab of NUnit console for any message saying something like this:
Failed to removeWhere
Verify file ownership and access permissions.
If you still want to use Shadow Copying support in NUnit, go to NUnit's console's settings dialog and specify a short directory as a Shadow Copy Cache, which is usually your "%Temp%\nunit20\ShadowCopyCache". Alternately turn off the Shadow Copy can help to alleviate this problem.