Update test_thinpool_expand to handle DATA_LOWATER > INITIAL_DATA_SIZE
Update test_thinpool_expand to handle DATA_LOWATER > INITIAL_DATA_SIZE

Call pool.extend_thin_data_device() after write loop has complted.

Signed-off-by: Todd Gill <7afbb5ba98932b8ffb9cc9e6675c4792db142529@redhat.com>

trgill/stratisd,mulkieran/stratisd,stratis-storage/stratisd,trgill/stratisd,stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd