test-tool: Refactoring residuals write tests

Looking at test_write10_residuals.c, test_write12_residuals.c and
test_write16_residuals.c tests the similarity of the testing scenario
can be found. There are several EDTL and SPDTL combinations which are
the same for different length write command tests. They form the core
of testing scenario of these commands. There aren't so much differences
in the way of testing these combinations itself either. It is possible
to move the main parameters describing the testing scenario into a
separate structure and move the scenario itself into a separate function.
It will increase the readability and reduce the duplicate code of these tests.

include/iscsi-private.h

@@ -37,6 +37,9 @@ extern "C" {
 #ifndef MAX
 #define MAX(a, b) (((a) > (b)) ? (a) : (b))
 #endif
+#ifndef ARRAY_SIZE
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#endif
 
 #define ISCSI_RAW_HEADER_SIZE			48
 #define ISCSI_DIGEST_SIZE			 4