[json] Added validation checks for generated XML/JSON

Enhanced assert methods in RestWriterTestBase to check, if the
generated XML/JSON is parseable.
Fixed some JSON tests that do not generate outter JSON object/array.
Fixed LinkGroupsConverter: anyonymous array inside an object is
not valid JSON.

Change-Id: I9c91f5c0aaf1267cf5f9e15e3ea65df49a7b56e5
Signed-off-by: Michael Ochmann <michael.ochmann@sap.com>
13 files changed