test: add test for sanitized notmuch-search output

This feature was recently added, so it of course needs a test now.

Signed-off-by: Jameson Graef Rollins <jrollins@finestructure.net>
Edited-by: Carl Worth <cworth@cworth.org> Fixed test to use
notmuch_search_sanitize in order to be robust against unpredictable
thread ID numbers, (due to unpredictable order in which the filesystem
presents files).