When the NOTMUCH variable was originally invented it was used as an
explicit path to the notmuch binary being tested. Today, the test
suite sets the PATH variable instead, so the NOTMUCH variable always
has a value of simply "notmuch".
We simplifying that by using the constant value rather than the
continual variable reference.
Now that we can usefully pass section names via the NOTMUCH_SKIP_TESTS
environment variable, it's useful to actually print those names out
for the user. Then, since we're now printing these names, let's use
nicer names, (not excessively long but also not using abbreviations
like "msg").
