test: new ideas; 'common.vim' refactoring started;

 * TODO:
  * added refactoring ideas;

 * test/base.vim:
  * new file to contain code to be used from 'common.vim' and
     'runutil.vim';
  * still without meaningful code;

 * test/common.vim:
  * initial changes to make the code easily moveable to 'base.vim';

 * test/runutil.vim:
  * small 'FIXME:' comment to show where we'd need to add a ':silent'
     command to avoid the underlying shell commands being displayed to
     the test framework user/caller;