test: refactoring: improved processor scripts protocol/use

 * test/runutil.vim:
  * added local ('s:'-local) debugging support;
   * for now, it does *not* use evlib#debug#DebugMessage();
   * added exception handling debugging messages;
   * modified code to use new functions (instead of the "manual"
      'echomsg' the code used before);
  * new function: s:EVLibTest_RunUtil_Local_GenUserScript_Source():
     generic (and reusable) way to call a user script to perform an
     operation, with well defined "input" and "output" variables;
   * new function: s:EVLibTest_RunUtil_Local_GenUserScript_ClearVars();
  * new function: s:EVLibTest_RunUtil_Local_ProcessorDef_Invoke();
   * 'processor'-based abstraction to invoke functions defined for a
      'processor' (script);
  * defined the protocol that the so-called 'processor' scripts have to
     follow;

 * test/evtest/proc/evtstd/v0-1-0.vim:
  * modified to implement new 'processor script' protocol;
   * only minor changes were needed;