Prevent tests that depend on Linux to run on OSX
Prevent tests that depend on Linux to run on OSX

Some basht tests depend on Linux to run. Actually they are being tested
by the pipeline, which executes `bundle exec rake spec:unit`. However,
when one executes that command on OSX workstations the test will fail
because it depends on some bash commands only present on Linux
distributions.

Since we are trying to make our tests be "locally runnable" I've added a
check to skip the tests when we run `bundle exec rake spec:unit` from
our OSX workstations

[#142582129]

pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release