In the original json code, search matching nothing would return a
valid, empty json array (that is, "[]"). I broke this in commit
6dcb7592e3 when adding support for
--output=threads|messages|tags. This time, while fixing the bug also
add a test to the test suite to help avoid future regressions.
When the NOTMUCH variable was originally invented it was used as an
explicit path to the notmuch binary being tested. Today, the test
suite sets the PATH variable instead, so the NOTMUCH variable always
has a value of simply "notmuch".
We simplifying that by using the constant value rather than the
continual variable reference.
The numbers were meaningless, and they made it hard to find a file of interest.
Instead, we get the ordering we want by adding an explicit list of
tests to run to the notmuch-test script.
