Problem:
  The test is searching for executable called mysqld only, while
  binary can be name mysqld-debug.

Reprod:
  Build with:
   $ cmake .. -G Ninja -DBUILD_CONFIG=mysql_release -DWITH_DEBUG=1

  and run test:
   $ ./mtr --debug-server --mem main.check_basedir

Fix:
  Get basename of binary from $MYSQLD and use that as name of binary.

Change-Id: I398f00d396110c30f8c7a9c95f3a1ec7dcbef81b