(Maint) use PuppetlabsSpec::PuppetSeams.parser_scope

(Maint) use PuppetlabsSpec::PuppetSeams.parser_scope

Without this patch all of the spec tests for parser functions in stdlib
would instantiate their own scope instances.  This is a problem because
the standard library is tightly coupled with the internal behavior of
Puppet.  Tight coupling like this creates failures when we change the
internal behavior of Puppet.  This is exactly what happened recently
when we changed the method signature for the initializer of
Puppet::Parser::Scope instances.

This patch fixes the problem by creating scope instances using the
puppet labs spec helper.  The specific method that provides scope
instances in Puppet-version-independent way is something like this:

 let(:scope) { PuppetlabsSpec::PuppetSeams.parser_scope }

This patch simply implements this across the board.