Unify header ordering in unit tests

Make sure all unit tests include headers in a similar order:

 1. Three headers which must be included before <cmocka.h>.
 2. System headers.
 3. UNIT_TESTING definition, followed by the <cmocka.h> header.
 4. libisc headers.
 5. Headers from other BIND libraries.
 6. Local headers.

Also make sure header file names are sorted alphabetically within each
block of #include directives.

lib/ns/tests/plugin_test.c

@@ -15,16 +15,15 @@
 #include <stddef.h>
 #include <setjmp.h>
 
+#include <limits.h>
 #include <sched.h> /* IWYU pragma: keep */
+#include <stdbool.h>
 #include <stdlib.h>
+#include <string.h>
 
 #define UNIT_TESTING
 #include <cmocka.h>
 
-#include <limits.h>
-#include <stdbool.h>
-#include <string.h>
-
 #include <isc/mem.h>
 #include <isc/platform.h>
 #include <isc/result.h>