flow test logs are suppressed

[jribbink]

As of cadence-tools/[email protected], logs are suppressed by default unless a logger is provided. These are useful diagnostics for developers and should be surfaced somehow.

I think the best solution is to output this to stderr, as it is fundamentally a user/diagnostic message, and this way it does not pollute the CLI output (i.e. previous issues have existed with this non-json data going to stdout when --output=json was used)