Test command with args

The default command line tester does not allow args on the comandLine node.
In some cases, to test the command, we need to pass args.
eg. Blender is a GUI application, to work headless, we need to pass the -b (background) argument.

Proposed solution (advanced by Florent Guillaume):

• Add to CommandLineDescriptor an optional testParameterString (also winTestParameterString) xml element;
• Add getTestParametersString() method;
• Change SystemPathExistTester to use these test parameters on command runtime exec.

It's now possible in the extension point for a command to include a <testParameterString>:

  <extension target="org.nuxeo.ecm.platform.commandline.executor.service.CommandLineExecutorComponent" point="command">
    <command name="mycommand">
      <commandLine>somecommand</commandLine>
      <parameterString>arg1 --arg2 --arg3</parameterString>
      <winCommand>somecommand.exe</winCommand>
      <winParameterString>arg1 /arg2 /arg3</winParameterString>
      <testParameterString>--exists</testParameterString>
      <winTestParameterString>/exists</winTestParameterString>
    </command>
  </extension>