(PUP-3091) Add test coverage for heredoc support

Before this commit the spec tests for heredoc support in Puppet did not
include tests to make sure that escaping new lines used the correct
margin on the preceding lines.

This commit adds in tests to ensure that we do not regress on this in
the future.
Merge pull request #3327 from demophoon/fix/stable/pup-3091-heredoc-support

(PUP-3091) Add test coverage for heredoc support