test: refactoring: moved constant to new file '.../c-defs.vim'

 * test/evtest/proc/evtstd/c-defs.vim:
  * new file: only one (important) constant in this file for now (to
     show feasibility and test the idea);

 * test/base.vim:
  * used constant defined in 'c-defs.vim';

 * test/evtest/proc/evtstd/v0-1-0.vim:
  * no longer "sourcing" 'base.vim': it now depends on 'c-defs.vim'
     only;
  * used constant defined in 'c-defs.vim';
  * removed old ("dead") code;