[buteo-sync-plugin-caldav] XML-sanitise .ics data. Contributes to JB#38601
Some servers don't sanitise the .ics data when returning it within an XML response. This previously caused QXmlStreamReader to error. This commit ensures we sanitise the input first.
It also improves build time by sharing intermediate build artifacts between the main plugin and the unit tests.
Contributes to JB#38601