Fix the look-behind test for OpenJDK

OpenJDK's regex engine can only handle look-behinds of limited sizes.
So let's just test for that, not the unbounded one we had before (that
our own regex engine handles quite fine, though).

This fixes issue #115.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #120 from dscho/look-behind-test

Fix the look-behind test for OpenJDK