(maint) Fix /import_export/import_export.rb Test

Prior to this commit, the /import_export.rb test was not passing for
Puppet Enterprise. This commit fixes that in a few different ways. First
the test itself needed to have its path modified for where the
puppetdb-import and puppetdb-export are located in PE. This was changed
by using the function created in the helper to determine the path. Next
the with_puppet_running_on function was used to check master status.
Finally the install_postgres function in helper.rb had to be modified
with module code.