Repair test-support publishing tasks

[rain-on]

Each test-support jar must be published with its own 'pom'
file, to ensure its dependencies are correctly documented.

As such, all test-support jars are now published under
tech.pegasys.pantheon.internal.testsupport (rather than being
stored in the same location as their main jar file).

[ajsutton]

LGTM because I can't think of a better option, but it's starting to feel pretty hacky. Would it be better to just put the DSL in a src/main/java package so it can be distributed normally?