testcommon: $ENV{TAIL} supports non-@ARGV redirects

Existing use of the $ENV{TAIL} relied on parsing --std{out,err},
which was only usable for read-only daemons.  However, -watch
doesn't use PublicInbox::Daemon code(*), so attempt to figure
out redirects.

(*) -watch won't able to run as a daemon in cases when
    git-credential prompts for IMAP/NNTP passwords.
    PublicInbox::Daemon is also designed for read-only
    parallelism where all worker processes are the same.
    Any subprocesses spawned by -watch are to do specific
    tasks for a particular set of inboxes.

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