Make PuppetSpec::Files methods available at module level.

Previously you could only access the tmpfile and tmpdir helpers by mixing
the module in somewhere, despite them exclusively mutating global state.

This became inconvenient when trying to centralize the database setup, so I
moved them to module functions and made the helpers that mix in defer to the
module level methods.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>