[REF] Fix import signature on activity parser, add preliminary test

[REF] Fix import signature on activity parser, add preliminary test

The construct class does not need to receive mapperKeys as a reference - this
was removed for Membership Parser here https://github.com/civicrm/civicrm-core/commit/14b9e069607c6cbcbbaf21a209f647ba422b8e04#diff-36f5db3555445d26df2de015a18598fd5d5682b76c62aee4f14dde95b7948274L65

In addition 2 places that instantiate this class pass in unused params (go copy & paste) so
that is removed. The test tests very little so far....