Make unit tests for DllImportGenerator build/run in CI

[elinor-fung]

• Rename DllImportGenerator.UnitTests to DllImportGenerator.Unit.Tests
  • Libraries test build looks for the .Tests suffix. Existing unit test projects in libraries are named .Unit.Tests, so I went with that (instead of making the build also look for .UnitTests).
• Make Ancillary.Interop build for minimum instead of current version.
  • Lets us compile our unit tests' code snippets against 6.0 reference assemblies.
• Set NUGET_PACKAGES environment variable to an accessible subdirectory when resolving ref assemblies.

cc @AaronRobinsonMSFT @jkoritzinsky

[jkoritzinsky]

Looks like we're going to need to point the NUGET_PACKAGES env var at another folder in the Helix machines to get the tests passing as is. Alternatively, we can move these tests to use the live ref pack by packing up the ref assemblies into a zip and sending them to Helix as well. I'm fine with either option for this PR, but we should do the second one at some point before shipping.

[elinor-fung]

Ergh, boo. Maybe that's part of why the other source generator tests explicitly list out all the specific implementation assemblies they need directly...

Trying the NUGET_PACKAGES path for this PR, since I expect we will continue to need that for our down-level unit tests.

Added #60605 to #60212.