Completes the shunit2 -> Hatchet test conversion.

These were the last shunit2 tests, so the testrunner, Makefile targets,
documentation and Travis CI steps have been removed.

There isn't yet a replacement Makefile target or documentation for the
Hatchet tests, however the rspec tests are easier to run by calling
rspec directly (compared to the in-Docker tests), and until it becomes
clearer what rspec use-cases are most common, it's hard to know how to
best structure the Makefile targets.

Closes @W-8786699@.

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