Clean up heredoc.t

* Made the tests more independent, mostly by decoupling the use of
  a single $string.  This will make it easier to expand on the test file
  later.

* Replace ok( $foo eq $bar ) with is() for better diagnostics

* Remove unnecessary STDERR redirection.  fresh_perl does that for you.

* fix fresh_perl to honor progfile and stderr arguments passed in
  rather than just blowing over them