sh: Add a test that manipulates various long strings.

It is quite effective at detecting mistakes in memalloc.c and code using it.

It is somewhat slow, but some of the patches in my queue improve it.

tools/regression/bin/sh/expansion/arith6.0

@@ -0,0 +1,16 @@
+# $FreeBSD$
+
+v1=1\ +\ 1
+v2=D
+v3=C123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
+f() { v4="$*"; }
+
+while [ ${#v2} -lt 1250 ]; do
+	eval $v2=$((3+${#v2})) $v3=$((4-${#v2}))
+	eval f $(($v2+ $v1 +$v3))
+	if [ $v4 -ne 9 ]; then
+		echo bad: $v4 -ne 9 at ${#v2}
+	fi
+	v2=x$v2
+	v3=y$v3
+done