Do not run unit tests older than v3

I did not add unit tests because it would require too much setup and a copy of NUnit v2 tests. I tested by attempting to run both v2 and v3 tests and combinations of the both. All work fine, no tests are found in the v2 assemblies.

The only thing I don't like about this is that it silently drops v2 tests. The only way I know to surface the error is through an exception, but that stops the entire test run.

I tried this logic in many locations, but this was the only place where it works. Everywhere else, the version 2 framework got loaded into memory causing a crash. Preventing v2 assemblies from even getting into the TestPackage seemed like the safest bet.