Is your feature request related to a problem? Please describe.
Currently, DXC lacks the capability to run tests with a specific environment variable set to allow for external validation. It also lacks the new class, DxcDllExtValidationLoader, which uses this environment variable. Though the new class hasn't been hooked up yet.

Describe the solution you'd like
The goal is to edit the cmake / lit files so that the check-clang lit test suite can be copied to separate instances and each of them get run, each time with a different environment, so that the environment has the new environment variable. Additionally, to keep this issue small and scoped, a test should be added to ValidationTest.cpp that verifies that the DLL is being loaded and older validation is being used when specified on the command line, without any special set up code.