test: Add simple tests for navigating notmuch-hello and notmuch-search views

We simulate the act of selecting the "inbox" saved search from
notmuch-hello and the act of selecting a desired thread from the
notmuch-search results.

The test for the navigation of notmuch-hello is currently marked as
BROKEN since its output is in the opposite order compared to the
'(notmuch-search "tag:inbox")' test. This question of ordering is a
currently open issue on the notmuch mailing list, so we'll let the
test suite reflect that for now.

Finally, this commit also abstracts some common emacs lisp code,
(waiting for the current buffer's process to complete), into a new
notmuch-test-wait function that is made available to anything calling
test_emacs.