By default TextTest runs your tests in parallel. Sometimes that’s not what you want.
In the static GUI there is a checkbox under the “running” tab where you can select “run tests sequentially”. This will apply to all test runs triggered while this checkbox is enabled.
If you want it to always run sequentially, add this line to your config file:
config_module:default
Why this works is a little obscure to me. The documentation for this config entry is here.