test: make test_emacs call post-command-hook

test: make test_emacs call post-command-hook

The unread/read changes will use the post-command-hook. test_emacs
does not call the post-command-hook. This adds a notmuch-test-progn
which takes a list of commands as argument and executes them in turn
but runs the post-command-hook after each one.

The caller can batch operations (ie to stop post-command-hook from
being interleaved) by wrapping the batch of operations inside a progn.

We also explicitly run the post-command-hook before getting the output
from a test; this makes sense as this will be a place the user would
be seeing the information.