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.
test_require_external prereq has to move to test-lib-common.sh, and
the new shell functions print_emacs_header and time_emacs are provided.
The somewhat indirect way of printing the output is to avoid the extra
"" present on string values from emacsclient.
