Fix UNUSED repetition issue in test sources (#475)

This is a follow-up PR to address UNUSED repetition issue (see
https://github.com/valkey-io/valkey/pull/446#discussion_r1593204956) in
different test source files.

Signed-off-by: Karthick Ariyaratnam <karthyuom@gmail.com>

src/unit/test_help.h

@@ -43,6 +43,8 @@
 
 #define TEST_ASSERT(_c) TEST_ASSERT_MESSAGE("Failed assertion: " #_c, _c)
 
+#ifndef UNUSED
 #define UNUSED(x) (void)(x)
+#endif
 
 #endif

src/unit/test_kvstore.c

@@ -1,5 +1,4 @@
 #include "../kvstore.c"
-#undef UNUSED
 #include "test_help.h"
 
 uint64_t hashTestCallback(const void *key) {

src/unit/test_util.c

@@ -4,7 +4,6 @@
 
 #include "../config.h"
 #include "../util.h"
-#undef UNUSED
 #include "test_help.h"
 
 int test_string2ll(int argc, char **argv, int flags) {