Are the projects/mbunit/gallio dlls of different .net runtime versions?
Try building them all against .net 3.5 framework.
Or, If all are targeted to .net 4.0, maybe try changing
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>
to:
<startup>
<supportedRuntime version="v4.0.30319" />
</startup>
in the test project's app.config