This used to be one of the features of GitHub - facebookarchive/xctool: An extension for Apple's xcodebuild that makes it easier to test iOS and macOS apps., and I would like to bring something like it back. swift test --parallel is really nice, and xcodebuild lacking something like that logic tests is definitely a hindrance to modular apps with split test structure.
I've toyed with the idea of just creating one big supermodule for tests across all our modules just to then use xcodebuild's single-target parallelization, but that's less than idea.