Add regression test for recent regex(3) breakage

BREs recently became prematurely sensitive to the branching operator, which
outright broke expressions that used it instead of failing silently. Test
that \| is matching a literal | for the time being.

Reviewed by: cem, emaste, ngie
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D11577