[rust] allow running tests in debug mode

## Description / 説明
running tests in release mode is hard coded into the tool https://github.com/online-judge-tools/verification-helper/blob/master/onlinejudge_verify/languages/rust.py#L236

I want the option to run tests in debug mode

## Motivation / 動機
debug mode catches things like integer overflow

-   Will you try to create a pull request?
    -   no
