simplify and fix performance tests

[Tomi Ollila]

On Wed, May 15 2013, david at tethera.net wrote:

> Jani noticed the alleged file notmuch-memory-test was missing.
> Austin grumbled about explicitly adding tests to a list.
> This series should give them both something else to grumble about.

This test series looks good to me. It is consistent what was there
before and easies future development.

If there is desire to handle the cases where executed test
scripts exit nonzero that can be done in future commits
(in any case).

+1

Tomi