Plain C dll is not copied by unit-test-runner

Hi,

I am working on a unit-test (VS2012/MS-Test/ReSharper-TestRunner) to verify a class  accessing a plain C dll (which is a interface to a external program).
I  have added this dll to my project and marked it as "Copy to output  directory : Copy always".
Also I added  the Attribute [DeploymentItem("<my dll>")] to:
[ClassInitialize]
[DeploymentItem("<my dll>")]
MyTestClassInitialize()
{
}

Unfortunately the dll is not copied as expected causing my unit tests to fail.
What is it I am doing wrong?

Regards
Rainer