regexp: fix TestOnePassCutoff
regexp: fix TestOnePassCutoff

The stack blowout can no longer happen,
but we can still test that too-complex regexps
are rejected.

Replacement for CL 162770043.

LGTM=iant, r
R=r, iant
CC=bradfitz, golang-codereviews
https://golang.org/cl/162860043

matloob/regexp,matloob/regexp,matloob/regexp