because searching for them makes the script abort with all non-NDB builds.


mysql-test/mysql-test-run.pl:
  Checking with "mtr_exe_exists()" for the location of a binary will yield an error
  if the binary is found nowhere at all, and this happens with NDB binaries in all
  builds that do not contain NDB;  so the correct way is fixed path names only.
  Still part of the fix for the #21721 bug.