(QENG-1404) Segregate system testing gems

Prior to this there was generic :test group.

Unfortunately Beaker will be EOL-ing support for Ruby 1.8 (a number of
Beaker's dependencies already have and pinning to older versions is
becoming costly). Once Beaker does this it will cause failures whenever
running `bundle install`.

To avoid this failure we can segregate the system testing gems, allowing
unit, lint and development to continue with
`bundle install --without system_tests`.