These tests deliver all possible (single-root) four-message threads in
all possible orders and check that notmuch successfully links them
into threads. These tests supersede and replace the previous and much
less thorough "T260-thread-order" tests.
There are two variants of the test: one delivers messages that
reference only their immediate parent and the other delivers messages
that reference all of their parents. The latter test is currently
known-broken.
