mysql-test-run --manager --valgrind
(without this fix, the manager fails to start mysqld and the tests hang).


mysql-test/mysql-test-run.sh:
  When running with --manager: the MySQL manager wants the complete path of the
  executable (it uses execv(), not execvp(), so does not search in the $PATH,
  so telling him to start 'valgrind' is not enough, it wants '/usr/bin/valgrind'
  or so).
  So this is a fix to be able to
  mysql-test-run --manager --valgrind
  Plus a warning (previously, if valgrind was not installed, tests silently
  hanged when run with --valgrind).