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/isc/tests/regex_test.c

@@ -20,13 +20,13 @@
 #include <string.h>
 #include <unistd.h>
 
-#define UNIT_TESTING
-#include <cmocka.h>
-
 #ifdef HAVE_REGEX_H
 #include <regex.h>
 #endif
 
+#define UNIT_TESTING
+#include <cmocka.h>
+
 #include <isc/commandline.h>
 #include <isc/regex.h>
 #include <isc/print.h>