made ntape tests sequential. seems there is some internal state which goes wrong during parallel execution (skolemization is a possible candidate, since it is not threadsafe, but there must be sthg else too).

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@2363 85d21ef8-add0-11de-ae77-5bfb9c5a3f34