test suite: add more debugging output for notmuch-new

[David Bremner]

The first patch in the series is a trivial cleanup.

I _thought_ (and tricked Tomi ;)) that the next one was trivial too,
but now I'm not so sure.

First, I forgot to update one test, but I mostly understand (or can at
least rationalize) that output. Then I had the bright idea that I
should add debugging output to other tests in T050-new. In 5 cases I
can't really understand the results. It seems like it takes several
runs of notmuch-new to remove directory entries. Maybe this is
intended, but I couldn't figure that out in a quick look.