Tests: Asynchronous iSCSI read test

Dispatch a number of READ10 requests simultaneously, and await all
responses.

The iscsi-support test helper functions are currently all synchronous,
so this test mostly uses the bare libiscsi API. As new async tests are
added, we can make a more informed decision on which boilerplate code
can be split out.

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

test-tool/iscsi-test-cu.h

@@ -145,6 +145,7 @@ void test_read10_rdprotect(void);
 void test_read10_dpofua(void);
 void test_read10_residuals(void);
 void test_read10_invalid(void);
+void test_async_read(void);
 
 void test_read12_simple(void);
 void test_read12_beyond_eol(void);