Because of the way the parallel_tests gem invokes rspec, we need to make
sure we have generated a binstub within the module for rspec. Otherwise
when a user invokes `pdk test unit --parallel`, parallel_tests will
attempt to invoke rspec outside of the PDK-managed Ruby environment.

The package acceptance test suite for multiple puppet version support was
calling `pdk bundle update` which was causing non-packaged gems to be
downloaded into the user cachedir by PDK. This caused later test suites
to resolve onto those non-packaged gems which would cause unexpected
diffs in the module's Gemfile.lock.

This commit updates the multi-puppet test suite to use `pdk bundle
update --local` and also ensures that the PDK user cachedir is cleaned
between each test.