(#19727) Collapse multiple leading slashes

Puppet 2.6.5 to 2.7.19 collapsed multiple leading slashes. This behavior
was changed in 2.7.20 in commit 912ed346 as a result of no longer
calling `File.split`.

This commit reinstates the behavior that existed previously and adds
tests for both double and triple leading slashes.

Paired-with: Rob Reynolds <rob@puppetlabs.com>
Paired-with: Kylo Ginsberg <kylo@puppetlabs.com>
(Maint) Don't prefetch from /tmp/test

Previously, the parsed file provider was prefetching and reading from
/tmp/test, which failed if you happened to have a directory /tmp/test.

This commit updates the test to use a temporary and empty file from it
prefetches.

Paired-with: Rob Reynolds <rob@puppetlabs.com>
Paired-with: Kylo Ginsberg <kylo@puppetlabs.com>
Merge branch 'ticket/stable/19727-double-slash' into stable

* ticket/stable/19727-double-slash:
  (Maint) Don't prefetch from /tmp/test
  (#19727) Collapse multiple leading slashes