Fix the look-behind test for OpenJDK
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 <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

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