[test_utils] Fix a minor UB

[test_utils] Fix a minor UB

(UBSan occasionally finds something interesting and
often reports whacky non-bugs like this one. "Fixing"
it will make the real UB bugs easier to identify, so...)

According to C's integer promotion rules, `unsigned short` gets
promoted to _signed_ `int`, and shifting into the sign bit of an `int`
is technically UB.  Explicit cast to `unsigned` quiets UBSan.