clarify test case with strict-arith

spec/arith.test.sh

@@ -80,10 +80,15 @@ set -o strict-arith || true
s=foo
echo $s
echo $((s+5))
echo 'should not get here'
## status: 1
## stdout-json: "foo\n"
## N-I bash status: 0
## N-I bash stdout-json: "foo\n5\n"
## N-I bash STDOUT:
foo
5
should not get here
## END
## N-I dash status: 2
## N-I dash stdout-json: ""
## N-I mksh status: 1