Notes on writing tests for libPlasma

* stdout, stdin, and the test log

When tests are passing, there should be no output other than that of
the test framework.  The obvious way to produce this is to send all
possible output from inside the script to /dev/null.  However, it's
much more useful if we see the unexpected output when a test fails.
Also, we want to save the output of the scripts in a test log so that
we can look at it when something goes wrong.  How to achieve this?

Commands that should succeed: Redirect stdout to the test log:

  ./cmd >>${TEST_LOG}

Commands that should fail: Redirect stderr to the test log:

  ./cmd 2>>${TEST_LOG}

A nice side benefit of this is that you can detect bugs like writing
error messages to stdout instead of stderr.
