Non-package-inferred test suites

[mtstickney]

According to the readme, rove:run takes a package, and runs the test suite for that package, but in practice rove:run calls rove/core/suit:run-system-tests, which attempts to load the argument as a system name. This will fail for any package that is not part of a package-inferred-system ASDF system, which means rove can only be used with those.

If rove is exclusively for package-inferred systems, that's fine, but the docs should probably be updated to reflect that since it's not very clear now. If it's meant to support both, on the surface you could just check the result of asdf:find-sytem before trying to asdf:load-system it, but it looks like there's a lot of setup that happens in run-system-tests, and I'm not sure if that would need to be separated out to run non-system tests.