Repairing Mapfishapp Testsuite

This commit aims to re-introduce the broken mapfishapp testsuite, with some improvements:

- In some tests, JSON objects were built using a StringBuilder, this PR proposes to replace them with pure JSON API, supposed to be more reliable,
- Some tests have been disabled: Pure GeoTools vs OGR implentations can lead to produce different results, so checking both using the same assumptions makes it easy to break the suite. In addition, it is necessary to check that GDAL/OGR native libraries / bindings are reachable by the JVM process, which might not be the case,
- Some tests were relying on external WFS services (openplans.org), as well as a postgres database available locally. This PR mocks every remote objects to avoid this need,  making it easier to maintain in the future.

See https://ssl.beneth.fr/reporting/mapfishapp/surefire-report.html for more infos about the testsuite.