SharpDevelop 4.2 now has a new
sample addin that adds integrated support for running tests
with
MSTest.
Features
Given the following code:

In the Unit Tests window the MS Tests will be displayed.
The unit tests can be run by right clicking and selecting Run
tests. The Unit Tests window will update to show the
tests that passed and failed.

The output from the MS test runner (mstest.exe) will be
displayed in the Output window.

Any test failures are displayed in the Errors window.

There is a default path defined for mstest.exe which points to
the Visual Studio 2010 folder but you can alter this path from the
Tools Options dialog under Tools - MS Test.
Note that running tests with the debugger or with code coverage
does not work.
Full details on how the addin works can be found in the
wiki.
Using the MSTest addin
To use the MSTest addin either follow the instructions in the
wiki to build it from source or download the
pre-built addin and register it with SharpDevelop using the
Addin Manager by selecting AddIn Manager from the Tools menu.