test: support testing notmuch as installed

We put some effort into testing the built copy rather than some
installed copy. On the other hand for people like packagers, testing
the installed copy is also of interest.

When NOTMUCH_TEST_INSTALLED is set to a nonempty value, tests do not
require a built notmuch tree or running configure.

Some of the tests marked as broken when running against installed
notmuch are probably fixable.