Auto merge of #23127 - alexcrichton:bench-wrapping, r=brson

Right now the rust upgrade in cargo is blocked on fixing this overflow. If a
this benchmark is run it will trigger an overflow error today:

    #[bench]
    fn foo(b: &mut test::Bencher) {}

This commit adds a check on each iteration of the loop that the maximum
multiplier (10) doesn't overflow, and if it does just return the results so far.

diff --cc src/libtest/lib.rs
Simple merge