abis_test: Fix the test case and calculate the right offset

We parse the load_config, take the ptrdiff_t from start and load_config
and from the previous array as the alignment can differ on different ABIs.

This was found by Daniel when executing the tests on a 64 bit userspace.
diff --git a/openbsc/tests/abis/abis_test.c b/openbsc/tests/abis/abis_test.c
index 4942a1e..e7e78d2 100644
--- a/openbsc/tests/abis/abis_test.c
+++ b/openbsc/tests/abis/abis_test.c
@@ -129,11 +129,11 @@
 		abort();
 	}
 
-	printf("Start: %u len: %zu\n", descr[0].start - dual_config, descr[0].len);
+	printf("Start: %u len: %zu\n", descr[0].start - load_config, descr[0].len);
 	printf("file_id:  %s\n", osmo_hexdump(descr[0].file_id, descr[0].file_id_len));
 	printf("file_ver: %s\n", osmo_hexdump(descr[0].file_ver, descr[0].file_ver_len));
 
-	printf("Start: %u len: %zu\n", descr[1].start - dual_config, descr[1].len);
+	printf("Start: %u len: %zu\n", descr[1].start - load_config, descr[1].len);
 	printf("file_id:  %s\n", osmo_hexdump(descr[1].file_id, descr[1].file_id_len));
 	printf("file_ver: %s\n", osmo_hexdump(descr[1].file_ver, descr[1].file_ver_len));
 
diff --git a/openbsc/tests/abis/abis_test.ok b/openbsc/tests/abis/abis_test.ok
index ba1da33..2f99f9d 100644
--- a/openbsc/tests/abis/abis_test.ok
+++ b/openbsc/tests/abis/abis_test.ok
@@ -7,10 +7,10 @@
 Start: 13 len: 13
 file_id:  09 07 05 
 file_ver: 06 07 08 
-Start: 51 len: 26
+Start: 0 len: 26
 file_id:  31 36 38 64 34 37 32 00 
 file_ver: 76 32 30 30 62 31 34 33 64 30 00 
-Start: 77 len: 26
+Start: 26 len: 26
 file_id:  31 36 38 64 34 37 32 00 
 file_ver: 76 32 30 30 62 31 34 33 64 31 00 
 SELECTED: 1