(#12357) Fix broken compatibility with Puppet 2.6

(#12357) Fix broken compatibility with Puppet 2.6

Without this patch, the previous change set to the
validate_absolute_path() parser function contains Puppet 2.6
incompatible changes.  stdlib 2.x is compatible with Puppet 2.6.  These
changes are a problem because we cannot introduce backwards incompatible
changes in a minor release.

This patch fixes the problem by back porting the implementation of the
`Puppet::Util.absolute_path?` from 2.7.x to the function block itself.

The function block tests to see if `Puppet::Util.absolute_path?` will
respond and if not, falls back to the inline back ported implementation.

The spec tests have been updated to simulate the behavior of Puppet 2.6
even when running with Puppet 2.7.