NPerfRunner duplicates loaded tested types for a fixture

[ovidiomanteiga]

Steps to reproduce the bug (using NPerf.Fixture.IList):

1. Launch NPerfRunner WPF application.
2. Load C5.dll (nothing yet at the tree panel).
3. Load NPerf.Fixture.IList.dll (it loads 1 type from it and 6 types from C5). Still OK.
4. Load mscorlib.dll (it loads 4 types from it and duplicates the previously loaded types).
   Everything works well, but it must not duplicate already loaded types.

[ovidiomanteiga]

This bug is partially fixed by ovidiomanteiga@5e0ee33

After creating the PerfLab instance and assign it, commonData.Lab was not null, so the assembly was added twice.

It still duplicates previously loaded types after loading new assemblies.

[ovidiomanteiga]

This bug is finally fixed with this change in NPerf ovidiomanteiga/NPerf@d0ba4ad. See comments there.

Now all the tests are loaded correctly, without duplicates.