To pause a running disk surface test, please:

- click on the disk surface map (any of the small squares) to display the contents of the underlying sector. While this window displayed, the disk surface test is automatically paused.

or

- click on the Stop button which shows a confirmation dialog (and pause the disk surface test).

If you need to stop the disk surface test, you can any time start a new surface test and continue where the previous test stopped: please open Disk menu -> Surface test and select the appropriate hard disk drive / SSD and testing method.
Then before starting the test, select the Configuration tab in this window and enable Limit testing to specific data blocks. Under this option you can specify first block where to start the disk testing. For example, if you specify 4000, then the disk test will start from 40%, if previously the first 40% tested (as each block represents 0.01% of the complete usable data area).