bug#42713: srfi-64 : Reset test-runner-current if done

[Jérémy Korwin-Zmijowski]

Hello !

Some time ago, I reported a bug to Per Bothner about srfi-64. The
runner was not reset so the report was incrementing each time I ran the
tests in a REPL session.

Per commited a fix. I thought it could be applied to Guile's module. 

Here is the link to the details : 

https://github.com/scheme-requests-for-implementation/srfi-64/commit/7cf4c010398850b45f5a6939bb73f747605a0513

Kind regards,

Jérémy