tests: New helper function feed_input, for comment support

tests: New helper function feed_input, for comment support

feed_input drops lines starting with a bar character '|', so they can
serve as comments.  Syntax chosen because such lines shouldn't be
needed in tests.  In particular, the server already ignores such lines
when it reads commands, because they get parsed as empty command with
a pipeline, and empty commands get ignored, regardless of
redirections.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>