Use AdHocWorkspace for generator benchmarks

[chsienki]

    Can we avoid using MSBuildWorkspace for our benchmarks? I'd much rather use an AdhocWorkspace and Jared's assembly references nugets to more precisely control the environment.

Originally posted by @333fred in #7963 (comment)

[chsienki]

We're now seeing issues in CI with using MSBuildworkspace and so disabled the tests on CI, we need to switch over to AdhocWorkspace and re-enable them.

Note, this isn't as simple as just switching the workspace and using the assembly references, as the Razor source generator expects certain files (such as the generated msbuild editor config) to be present in order to function correctly, so we'll need to ensure that we can create those too.

We could possibly just evaluate the project and extract out the generated config, but we'll need to make sure we have a process for updating the tests from the on-disk project as needed.