The worry here is that a binary linking with libnotmuch might lose
access to Xapian::Error symbols because libnotmuch hides them.
We are careful here to create ./fakedb/.notmuch in order to trigger a
Xapian exception, and not just a missing file check.
Thanks to jrollins and amddragon for suggestions.
(cherry picked from commit 66f37f5f6864a988f94ddb893e3a176af57f6c8e)
