test: Test thread linking in all possible delivery orders

test: Test thread linking in all possible delivery orders

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.