mon: shell test helpers to run MONs from sources

mon: shell test helpers to run MONs from sources

The intent is to make it more convenient to reproduce a specific mon
behavior and observe the result by grepping the logs. It can be handy
for bug diagnostic. The test could be included as a unit test to
be run on make check.

The setup function will prepare a directory and kill leftover from a
previous run. The teardown function cleans up on success. The run
function is expected to be provided by the calling script and can make
use of the run_mon function to mkfs + run a monitor.

Signed-off-by: Loic Dachary <loic@dachary.org>