What is the easiest way to test multiple versions of a NuGet?

Question

I have two C# projects. Project A is a NuGet package. Project B is a web service. Project A calls the web service.

We want B (the web serice) to remain backward-compatible with old versions of A - i.e., old versions of A can still call the newest version of B.

I want to write integration tests to verify this. This is easy enough to test for a single version of A - just install the NuGet package in a test project and run some tests against an instance of the web service.

What is the most convenient way to do this for multiple versions of the NuGet? I cannot easily reference multiple versions of a NuGet and switch between them at runtime. Of course I can make multiple test projects and reference a different NuGet version in each test project, but do I then have to copy the tests themselves across multiple projects? That would be very inconvenient.

Is there a nice way to do this? Can I maybe do something clever with preprocessor directives and build different DLLs from the same source code? Or something?

Thanks in advance!