need to child.kill() when returning Status::Timeout in runner #146

I noticed a few comments along the lines of trying to avoid mutating into infinite loops. While of course this is good to avoid too many spurious too-weak mutations, it's kind of theoretically impossible in general. The current implementation leaks processes that consume 100% CPU forever.

It's easy to fix, as described in the title.

I ran into this with the following test case.

#[mutate]
fn finite_loop() -> usize {
    let mut x = Some(true);
    let mut n = 0;
    while x.is_some() {
        if x.unwrap() { // REPLACE_WITH_TRUE here triggers it
            x = Some(false);
        } else {
            x = None;
        }
        n += 1;
    }
    n
}