(maint) Remove FS redefined constant warnings in spec runs

We define FS as an alias for Puppet::FileSystem in three separate
specs. Unfortunately, describe blocks do not provide any namespacing in
rspec, and this were colliding, issuing two separate warnings for a
redefined constant.  Wrapping each spec in its own module provides
separation and elliminates the warnings.
Merge remote-tracking branch 'upstream/pr/2586'

* upstream/pr/2586:
  (maint) Remove FS redefined constant warnings in spec runs