add test for Util#writeAnnotation()

[vladak]

When working on #4761 it became evident that Util#writeAnnotation() is not tested. This tracks the addition of such tests.

[vladak]

In particular, the matching of description using revision should be tested, i.e.

opengrok/opengrok-indexer/src/main/java/org/opengrok/indexer/web/Util.java

Line 720 in 2a5d034

String revision = annotation.getRevision(num);

vs.

opengrok/opengrok-indexer/src/main/java/org/opengrok/indexer/web/Util.java

Line 735 in 2a5d034

String msg = annotation.getDesc(revision);