publish a test artifact too

partially addresses #561

note that for now the artifact contains only test classes, not any
test resources.  so this commit isn't as spiffy as the headless branch
equivalent in b24195a7911b90e18a3215dee42e09f9a5300310 and
214f94fddd83bf7184e3d2268058aa02f3e4990c.  it could always be further
improved later.