When opening a new connecgtion, changed mysqltest to prefer a brand new connection slot over an existing, closed slot. Fixes a problem with reused slots that can cause tests to fail. bug#29579


client/mysqltest.c:
  When opening a new connecgtion, changed mysqltest to prefer a brand new connection slot over an existing, closed slot. Fixes a problem with reused slots that can cause tests to fail.