Finally I just changed all project in my test solution to x86-target.
There is that there are no exceptions thrown in case of 'Any CPU'-target projects. The Click() method just does not work.
I think it would be useful to make a post somewhere on your site about writing tests using NUnit framework.
I saw several posts decribing the same issue (about making tests with C#). To make your tests work you need to do:
1) Projects with tests should be 'x86'-targetted.
2) For a successful run of this tests use 'nunit-x86.exe' or 'nunit-console-x86.exe'. Tests will not run with 'nunit.exe' or 'nunit-console.exe'.