test-ether-addr-util: add a simple test that HW_ADDR_TO_STR works with nesting

It seems to, but I was a bit incredulous… The comment is adjusted to match
the standard.

The trick with a temporary buffer is neat. I wasn't sure if it is valid, but
the standard says so. The test really tests that we are reading the rules right
and that the compiler doesn't do anythign strange or emit an unexpected
warning.

src/basic/ether-addr-util.h

@@ -23,7 +23,9 @@ struct hw_addr_data {
 #define HW_ADDR_TO_STRING_MAX (3*HW_ADDR_MAX_SIZE)
 char* hw_addr_to_string(const struct hw_addr_data *addr, char buffer[HW_ADDR_TO_STRING_MAX]);
 
-/* Use only as function argument, never stand-alone! */
+/* Note: the lifetime of the compound literal is the immediately surrounding block,
+ * see C11 §6.5.2.5, and
+ * https://stackoverflow.com/questions/34880638/compound-literal-lifetime-and-if-blocks */
 #define HW_ADDR_TO_STR(hw_addr) hw_addr_to_string((hw_addr), (char[HW_ADDR_TO_STRING_MAX]){})
 
 #define HW_ADDR_NULL ((const struct hw_addr_data){})