More cgptlib tests

Add some extra cases to SanityCheckTest() to test both header and entries
being garbled at either end of the disk.

Add DuplicateUniqueGuidTest() to check that GPTs having duplicate
UniqueGuids in the entries are rejected. We can only check this per-disk, of
course.

Made some changes to the library to enforce the UniqueGuid requirement that
I just started testing for.

BUG=chromium-os:4854

Review URL: http://codereview.chromium.org/3135044

Change-Id: I86458faf9cc99aa3f29aac0d5b144dbd05067181

firmware/lib/cgptlib/cgptlib_internal.c

@@ -152,6 +152,10 @@ int CheckEntries(GptEntry* entries, GptHeader* h) {
       if ((entry->ending_lba >= e2->starting_lba) &&
           (entry->ending_lba <= e2->ending_lba))
         return 1;
+
+      /* UniqueGuid field must be unique. */
+      if (0 == Memcmp(&entry->unique, &e2->unique, sizeof(Guid)))
+        return 1;
     }
   }