The sources for the tests should be under ILVerify\tests as .il files, and they should get compiled as binaries. (BTW: ilasm has /ERROR option that suppresses some of its error checking that would otherwise prevent invalid test cases from compiling.)

Related tests should live in the same .il file, so that we end up with like a few dozen .il files at the end. I do not think we would want to have thousands little .il files as tests (slow builds, slow sync times, ...).

We tend to like to use XUnit as the test driver. Maybe [Theory] with MemberData generator will work well for this?

A good place to take the inspiration from would be \src\ILCompiler.TypeSystem\tests in the CoreRT repo: that's where we keep the type system tests. The type system tests have similar characteristics - each test loads an assembly we built from IL as part of the repo build and does stuff with it.