Test empty captures in p5regex.
Test empty captures in p5regex.

Includes a failing test for when one is at the start, which breaks
scanning.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp