Include <inttypes.h> whenever including <cmocka.h>

Development versions of cmocka require the intmax_t and uintmax_t types
to be defined by the time the test code includes the <cmocka.h> header.
These types are defined in the <stdint.h> header, which is included by
the <inttypes.h> header, which in turn is already explicitly included by
some of the programs in the tests/ directory.  Ensure all programs in
that directory that include the <cmocka.h> header also include the
<inttypes.h> header to future-proof the code while keeping the change
set minimal and the resulting code consistent.  Also prevent explicitly
including the <stdint.h> header in those programs as it is included by
the <inttypes.h> header.

tests/isc/buffer_test.c

@@ -12,6 +12,7 @@
  */
 
 #include <fcntl.h>
+#include <inttypes.h>
 #include <limits.h>
 #include <sched.h> /* IWYU pragma: keep */
 #include <setjmp.h>