Embedded Manifest with COM Free registration not working with NUNIT

I have an application which has an embedded manifest. I use COM free registration to make use of a 3rd party COM dll. The application works without errors. However when using NUNIT for integration tests, it throws:

Retrieving the COM class factory for component with CLSID {GUID} failed due to the following error 80040154 Class not registered (Exception from HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG))

Which seems to suggests that running the tests the embedded resource is not read/loaded for the COM free registration. Any suggestions to resolve this?