We have two test projects in a solution and the test list shows all tests from both projects.
I remember that I encountered this issue in the past where some test methods were missing from the test list and i suspect that it has to do whith the bug concerning the .vsdmi files. It can happen if your working with source control and allow multiple check out.
Make sure that:
All your test classes have the proper attribute.
All your methods have the proper attribute.
If it still happens backup you .vsdmi file put it somewhere else and delete the one(s) that are in your project tree (most likely the root). Then close and reopen your project/solution and hit Test/Window/Test View...
Hope this helps!