That step really requires that Tails is running to make sense, so any
scenario using it otherwise is incorrect.

We don't need the interactive progress report written to STDERR, and
we don't want to pollute it any more since we have this cucumber
formatter based logging.

It's always the same (=> useless) any way.

Actually this overwrites cucumber's 'debug' formatter, but it's
useless for anything except developing new formatters, which we might
as well do in a cucumber environment outside of Tails.

Will-fix: #9491

... because CRuby's threading support sucks when IO is involved.

The idea is to replace each instance where we print debugging info to
STDERR if $config["DEBUG"] is set, with a call to debug_log(), which
will make the pretty_debug formatter print the output *if* it is
actiaved.

This approach is cleaner than what we do now, and will make it
possible to e.g. usw the pretty formatter for the terminal output and
at the same time logging pretty_debug to a file. Upon error, the full
log can then be consulted, without making the terminal output
messy. While debugging/developing the test suite, the pretty_debug
formatter can still be directed to the terminal, of course.

A notable problem is how to deal with the various subprocesses/threads
whose STDERR output we also are interested in when debugging only,
like Sikuli's debugging info. We'll have to find a way to capture such
output and hook it into debug_log() some how.

... by moving the extra hooks into a null formatter that we always
activate. This also fixes a bug: if the first formatter was specified
twice (e.g. --format pretty --format pretty --out /tmp/test.log) then
the extra hooks would be run twice.

It's been removed with #9597.

Fix-committed: #9890

Fix-committed: #9877

... so that we won't be nagged when testing Tails 1.5.1 (in
particular, that would break large parts of the automated test
suite). Apparently this was forgotten during the 1.5 release. Oops!

Fix-committed: #10112