python3 doesn't allow dictionaries to be initialized with non-string
keywords. This presents problems on systems in which "python" means
"python3". We instead initalize the dictionary using the dict
comprehension and then update it with the values from the tree. This
will work with both python2 and python3.
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.
