test/multipath: add COMPARE AND WRITE test

Confirm that data is consistent across paths when using the atomic
COMPARE AND WRITE operation.
This should be extended in future to issue the requests simultaneously
across both paths, in an attempt to trigger a read/write race.

Signed-off-by: David Disseldorp <ddiss@suse.de>

test-tool/iscsi-test-cu.h

@@ -302,5 +302,6 @@ void test_writeverify16_residuals(void);
 
 void test_multipathio_simple(void);
 void test_multipathio_reset(void);
+void test_multipathio_compareandwrite(void);
 
 #endif	/* _ISCSI_TEST_CU_H_ */