make tests that didn't use TemporaryFolder use it instead of ad-hoc tmp folders
Exclude geogit-geoserver tests, they don't work properly with -Djava.io.tmpDir

The tests inherited from geoserver tests don't work properly if
java.io.tmpDir is specified (geoserver test infrastructure issue),
but this module is about to be removed anyways so no harm in disabling
the tests.
Allow to specify the tmp dir used for tests with mvn -Dtest.tmpDir=<path> ...