If any E2E test hangs, it'll eventually timeout and panic, causing the
all the remaining tests to fail. External commands should use a short
context whenever possible so we can fail the test quickly and move on
to the next test.
The E2E suite exercises the API, but not the CLI. This changeset adds a helper
function to send commands via a locally-built Nomad binary (which we'll need
to add to the E2E setup), and some helpers to parse the resulting structured
outputs in a way that tests can consume.
