add support for running unit tests in binary suite dependencies

[dougxc]

Recently the graal-core gate switched to importing the Truffle suite as a binary dependency. This had the unfortunately side effect of stopping the Truffle unit tests from being run on Graal in the gate. To fix this we need:

• Truffle to export its unit tests as a distribution (like Graal does).
• mx unittest command to look in distribution jars for tests