Add function mtr_exe_maybe_exists to look for binaries that is optional
Skip ndb support if it's a binary dist where mysqld supports ndb but there is no ndbd


mysql-test/lib/mtr_misc.pl:
  Add function mtr_exe_maybe_exists
mysql-test/mysql-test-run.pl:
  Adapt to running from binary dist
  Add function mtr_exe_maybe_exists to look for binaries that is optional
  Skip ndb support if it's a binary dist where mysqld supports ndb but there is no ndbd