Changed function 'warn' to macro '@warn' in runtests.jl

[macd]

runtests.jl for the SHA module uses the nonexistant warn function. Changed that to the @warn macro call.

[StefanKarpinski]

@c42f: would you mind reviewing this?

[c42f]

Well I'm a bit late to the party but FWIW it looks fine to me. Straightforward translation like this should always be possible for warn->@warn with a string argument.