Add a knob to allow tasks testing using ruby_task_helper

The puppetlabs/ruby_task_helper module provide helpers to write testable
bolt tasks.  For this to work, the tested module depends on a copy of the
ruby_task_helper module to be beside it: quoting ruby_task_helper's
documentation:

> Make sure the task helper repo is checked out next to your module so
> the relative requires work and you can write simple unit tests for the
> methods of your task.

This commit test for a `use_ruby_task_helper` setting to create a
symlink before running the test suite.  This link points to the fixture
directory where the appropriate version of ruby_task_helper will be
put according to what the `.fixtures.yml` file says in the module under
test.