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.
This logic should really be in the driver, but requires changes to how the driver is created.