test: refactoring: performance improvements (redir to variable support)

 * test/base.vim:
  * redirection: added support for redirecting to registers and
     variables;
  * TODO: left some comments related to function and variable renaming
     to better describe the new semantics (most of those still refer to
     "redirection to file(s)");

 * test/runutil.vim:
  * performance improvement: reduced file I/O by redirecting "test
     context info" to a variable;
   * ... and changing the way that tests do redirection when running in
      this mode (tests overwrite the "test output" file);
   * for the moment, this is enabled through a local variable (and
      we'll possibly remove the legacy "redirect the caller's instance
      to a file, too" functionality in a later commit);
    * variable name: l:test_output_runutil_redir_use_variable;
  * added a few more 'FIXME:' comments, to be implemented/addressed
     later;

 * test/testrun.vim:
  * added support to react to the added configuration variable
     'g:evlib_test_outputfile_truncate' (specified to be non-zero when
     the invoking instance has been running with
     l:test_output_runutil_redir_use_variable != 0 );