Seems like SimpleTest is designed to run PHPUnit tests locally. It would be nice to be able to run tests on remote machine through custom test runner script.
The plugin does not support running PHPUnit tests any more -- the code to run PHPUnit was for a very old version of PHPUnit and has not worked for several years.
Frankly speaking - it doesn't matter. If there will be API in plugin that will allow to send request for listing test names for some specified resource from workspace and then sending request to execute specific test or group of tests and if there will be agreement on returning result (JUnit XML?) - we won't even have to know if it was SimpleTest, UniTest or some other CustomTest or WebTest.
Development on the plugin has essentially stopped and this feature is way outside the scope of the plugin. This functionality is almost worthy of its own project. This requires some sort of negotiated protocol for a client (an eclipse plugin) to talk to some sort of listener service which will run one or more tests it is told to run. I would bring this concept up with one of the continuous integration projects -- they would have a need to run tests in this way. I am not even sure simpletest has an api which allows for execution of tests in this manner.
It's a pity that development is stopped. That can't be helped then even though the functionality is just a different source for PHP test result data.