Set QCHECK_MSG_INTERVAL to avoid clutter in CI logs

[jmid]

QCheck by default prints updates to stdout often to give the user a sense of progress.
This works well in a terminal/console, but less well in an append-only CI-log where the many printed lines adds clutter.
By setting the environment variable QCHECK_MSG_INTERVAL to something more sensible, we obtain cleaner CI logs.

(We also use this in multicoretests and domainslib - but it must have been an oversight in lockfree)

[lyrm]

Thanks !