Introduce versioning for linker tests

We're slowly starting to introduce behavioral differences based on the TFM of the app being linked. This is mostly to maintain backward compatibility without a need to ship different linker versions in the SDK. But we don't have any real test coverage for these cases.

https://github.com/mono/linker/blob/a621b9a630950026f5508c8089a59c0e825bd6e0/src/linker/Linker.Steps/MarkStep.cs#L1595
https://github.com/mono/linker/pull/2032/files#diff-f3ab7d627296ba105613b9cc039ca6f4ddc7a9aa66c6060ca82f6456ae0ede4fL1700

We should introduce a new test leg (at least in the CI) to run linker against lower versions of TFMs and have tests adapt accordingly.