Stop printing log messages on stderr

[tperami]

I'm using qtest as part of dune and it is very annoying for me that qtest prints some log output on stderr when everything is going normally. For example

Extraction : `src/modulename.ml' Done.

Either this output should be entirely suppressed or a --quiet option could be added to suppress it.

[c-cube]

would you mind testing on master with --quiet?

[tperami]

It stopped printing Extraction : and Done. but it still prints the name of the processed files in between.

[c-cube]

my bad. should be fixed now.

[tperami]

Seems good to me! Thank you very much!