Add Mocha to the Gemfile

Other libs/apps may use the test helpers herein. At present,
this gem imposes no Bundler requirement that Mocha be installed
prior to use, which causes, in particular, the organisations
test helper to fail on Mocha calls in libs/apps that aren't using
Mocha. Suspect this is working by coincidence elsewhere;
it may be necessary to review the test_helpers for hidden
dependencies - many seem to at least require 'plek'.