[Validation] Test validation behavior and add validation testing infrastructure

[bob80905]

Is your feature request related to a problem? Please describe.
Currently, there does not exist an adequate infrastructure to test the validator that is chosen when validating a shader.
A user can manually test which validation version is used when validating a shader, but there's no automated way within the DXC repo to do this testing.

Describe the solution you'd like
Testing infrastructure should be added to the DXC repo to validate that the right validator is being used in all of the following 3 cases:
-select-validator internal
-select-validator external
<no option, aka default behavior>

One approach is to add an environment variable that will point to a DXIL.dll to use for validation, if -select-validator external is specified. Then, a set of versioned DXIL.dlls can be added to the DXC repo, and a testing suite can be created with an environment variable that points to each of the stored DXIL.dlls. This way we can validate that the external option indeed selects the right validator.
The design should conform to the behavior described in https://github.com/microsoft/hlsl-specs/blob/main/proposals/infra/INF-0004-validator-hashing.md

[damyanp]

#7397

[damyanp]

This proposal describes a plan to address this: microsoft/hlsl-specs#499