TestGenerator should log warnings when the same class is tested

[Keboo]

Reproduction steps:
Put the ConstructorTestsAttribute on top of two test classes and set the same target type for both of the attributes.

Observed:
The output window properly shows an error that the generator produced duplicate files with the same name.

Expected:
There are two things that should be fixed.
First, we should handle two target classes that are in different namespaces but have the same class name. This should be fixed by making the output file names unique.
Second, we should log a warning if the ConstructorTestsAttribute is declared twice for the same target type.