Updatesite: Include integration-tests in Gitlab CI

[mchlrch]

Building the updatesite in gitlab CI.

Current situation: run only mvn package. Runs only unit-tests, no integration-tests

Goal: run mvn integration-test

Xtext integration-tests launch an OSGi runtime (an Eclipse) and execute the project's tests in that runtime (for details, see tycho-surefire:test).

To get this working requires an appropriate docker image that includes xvfb (an in-memory display server) or similar.

Some pointers:

• https://github.com/eclipse/xtext/issues/1323
• https://bugs.eclipse.org/bugs/show_bug.cgi?id=540236
• https://www.eclipse.org/lists/cbi-dev/msg01957.html

[mchlrch]

See also here: https://github.com/zazuko/prefix-server/blob/master/.ci/test.yaml#L26