I make cross-platform and multi platform tools. They for example load different modules depending on the runtime platform.
Obviously, I won't be able to fully test them in Visual Studio on Windows, but I could at least test if they detect the runtime platform correctly and if they load appropriate modules.
The system checks are pretty standard:
if (RuntimeInformation.IsOsPlatform(OSPlatform.Linux)) { /* do the Linux thing */ }
I test Linux code just by publishing a Linux executable (as standard console application) and then just output the test results to the console.
It would be cool if I would be able to add some XUnit tests to my Visual Studio solution that test some of this code.