Support compiling common source and platform source

[vRallev]

Right now expect / actual code cannot be tested because all source files are treated part of the same source set. The original Kotlin compile testing library has a hook to change that: https://github.com/tschuchortdev/kotlin-compile-testing/blob/a0e418b04f6113396c515ca36b8d41f86f9b7317/core/src/main/kotlin/com/tschuchort/compiletesting/SourceFile.kt#L15

[ZacSweers]

PRs welcome, this is pretty untraveled ground :/