Multitarget unit test projects not found

[georghinkel]

(this is somehow related to #187 but I am creating a new issue because my problem is more specific)

I found that AxoCover does not find unit tests from unit test projects targeting multiple platforms. For example, my unit test projecst using MSTest usually target .NET 4.5, .NET 4.6.1, .NET 4.7.1 and .NET Core 2.0. I can see my tests in the test explorer (once), but don't see them in Axo.