This is a bit more involved than replacing the use of
notmuch_database_open_verbose, as we have to effectively inline the
definition of notmuch_database_open.
If we're building against a version of Xapian that doesn't offer
retrying the lock, we should be honest and describe the tests as
broken, rather than marking them as missing a test prerequisite.
missing test prerequisites should be for specific components of the
test harness that are missing, not for the backend library notmuch
uses.
