Tests: Asynchronous iSCSI write test

Dispatch a number of WRITE10 requests simultaneously, and await all
responses.

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

test-tool/iscsi-test-cu.h

@@ -241,6 +241,7 @@ void test_write10_0blocks(void);
 void test_write10_wrprotect(void);
 void test_write10_dpofua(void);
 void test_write10_residuals(void);
+void test_async_write(void);
 
 void test_write12_simple(void);
 void test_write12_beyond_eol(void);