Fix lexical misunderstandings in gnus-icalendar-tests.el (bug#39782)

Fix lexical misunderstandings in gnus-icalendar-tests.el (bug#39782)

* test/lisp/gnus/gnus-icalendar-tests.el:
(icalendar-tests--get-ical-event): Remove unused function accidentally
copy-pasted from icalendar-tests.el.
(gnus-icalendar-parse, gnus-icalendary-byday):
Remove unintended initial newlines.
Duplicate comma-escaping backslashes so that they have intended
effects, conforming to RFC 5545.
Remove ineffective comma-escaping backslashes where not intended.