Change #!/bin/bash at start of tests to "#!/usr/bin/env bash". That way
systems running on bash < 4 can prepend bash >= 4 to path before
running the tests.
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.
