When creating $THREADS data it may end of not having 'None' at all
or the numbers in line output yields a loop.
To avoid loop the value in current array index is set to 'None'
so that if the same item is reached again the loop will end.
Also empty string as next array index will end the loop.
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.
All test scripts to be executed are now named as T\d\d\d-name.sh,
numers in increments of 10.
This eases adding new tests and developers to see which are test scripts
that are executed by test suite and in which order.
