test: add simple multipath IO test

Write data to device via one path, then read back using a different
path. Confirm that data is correct.

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

test-tool/iscsi-test-cu.h

@@ -283,4 +283,6 @@ void test_writeverify16_flags(void);
 void test_writeverify16_dpo(void);
 void test_writeverify16_residuals(void);
 
+void test_multipathio_simple(void);
+
 #endif	/* _ISCSI_TEST_CU_H_ */