Add testing for MSBuild targets

https://github.com/dotnet/linker/pull/2837 moved the linker targets to this repo, but testing for the logic in the targets is still being done in the SDK repo. We should at least add a basic smoke test to catch issues before changes flow to SDK - to prevent issues like this: https://github.com/dotnet/sdk/pull/26993#issuecomment-1209710886.