We should use different mount point / testpool for ubuntu_zfs_xfs_generic test

If there is any test failed for some reason, the mount point and test pool will be occupied.
And it will cause failures in the following tests:

 Creating zfs pool testpool..
 cannot create 'testpool': pool already exists
 Creating zfs file systems test and scratch in testpool..
 cannot create 'testpool/test': dataset already exists
 cannot create 'testpool/scratch': dataset already exists
 umount: /mnt/test: target is busy.
 cannot unmount '/mnt/test': umount failed
 VDEVs in /home/ubuntu/autotest/client/tmp/ubuntu_zfs_xfs_generic/src
 mkdir: cannot create directory '/mnt/test': File exists
 FSTYP -- zfs
 PLATFORM -- Linux/s390x s2lp6g001 5.0.0-14-generic
 MKFS_OPTIONS -- testpool/scratch
 MOUNT_OPTIONS -- testpool/scratch /mnt/scratch

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