How do I run a single UnitTest from rocket-chip?

Question

Specifically I'd like to run AXI4XbarTest from rocket-chip/src/main/scala/amba/axi4/Xbar.scala. It looks this test should be run by the regression tests, but if I go into the regression directory and run

export SUITE=UnittestSuite
make emulator-ndebug

I end with the error:

make VM_PARALLEL_BUILDS=1 -C /home/ben/Code/rocket-chip/emulato/generated-src/freechips.rocketchip.unittest.AMBAUnitTestConfig -f VTestHarness.mk
make[2]: Entering directory '/home/ben/Code/rocket-chip/emulator/generated-src/freechips.rocketchip.unittest.AMBAUnitTestConfig'
make[2]: VTestHarness.mk: No such file or directory
make[2]: *** No rule to make target 'VTestHarness.mk'.  Stop.

But really, I don't want to run the regression tests, I just want to run a single unit test, and I'm having trouble extracting how to do that from the Makefiles.