Avoid old C-style casts

[UnePierre]

My IAR C++ compiler (EWARM8.40) issues a remark on constructs such as (void)0:

Remark[Pa137]: use of C-style cast [...]

Of course, this can be silenced in a doctest-trampoline header, but so far, it is the only remark that needs this.
I'd like to point out this feature of doctest: it is really silent warning/remark-wise! :-)

Pull-request #425 should fix that.

[onqtam]

Closing this now that the PR has been merged.