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

contrib/netbsd-tests/lib/libc/regex/data/subexp.in

@@ -11,6 +11,9 @@ a(b+)c		-	abc	abc	b
 a(b+)c		-	abbbc	abbbc	bbb
 a(b*)c		-	ac	ac	@c
 (a|ab)(bc([de]+)f|cde)	-	abcdef	abcdef	a,bcdef,de
+# Begin FreeBSD
+a\(b\|c\)d	b	ab|cd	ab|cd	b|c
+# End FreeBSD
 # the regression tester only asks for 9 subexpressions
 a(b)(c)(d)(e)(f)(g)(h)(i)(j)k	-	abcdefghijk	abcdefghijk	b,c,d,e,f,g,h,i,j
 a(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)l	-	abcdefghijkl	abcdefghijkl	b,c,d,e,f,g,h,i,j,k