I have a c# solution with the following structure:
mySolution
myProject
myProject.MSTests
References
Microsoft.VisualStudio.QualityTools.UnitTe
Possibly a bit late, but this question googles up well, I thought I'd throw some crumbs in for future googlers.
Bryan Cook suggests checking the ProjectTypeGuids in his blog post about Manually creating a MS Test Project. Apparently the magic GUIDs you need are {3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} for c# and {3AC096D0-A1C2-E12C-1390-A8335801FDAB};{F184B08F-C81C-45F6-A57F-5ABD9991F28F} for VB. See his blog post for more details.
In case the blog post ever goes away you need to add the following element in the main property group in the csproj file:
{3AC096D0-A1C2-E12C-1390-A8335801FDAB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}