Move Contents of Prepare Filesystem into Simpletest Plugin

+1. PrepareFilesystem assumes that the containers are available anyway, so it shouldn't be in environment:

It looks like PrepareFilesystem doesn't even use FileHandlerTrait.

There's also this commented code that should end up uncommented:

//    if ($result !== 0) {
//      // Directory setup failed threw an error.
//      $this->io->drupalCIError("Prepare Filesystem failed", "Setting up the filesystem failed:  Error Code: $result");
//      throw new BuildTaskException("Setting up the filesystem failed:  Error Code: $result");
//    }
    //return $result->getSignal();

Simplifying the scope back to what it was. :-)

Chatting in IRC we decided the filesystem set up should be encapsulated in the simpletest plugin.

We thought SimpletestD7 might just override the prepareFilesystem method, but it requires the setup, as well.

Starting phantomjs and php -v are still present in prepare_filesystem, which shouldn't really be called prepare_filesystem any more. The PHP version part will get moved away in #2843549: RunContainers should output php -i as an artifact. We can deal with where to put the phantomjs startup in another follow-up.