Rename 'arena_decay' to 'arena_util'

While initially this file contained helper functions for one particular
test, now its usage spread across different test files. Purpose has
shifted towards a collection of handy arena ctl wrappers.

test/unit/arena_decay.c

@@ -1,5 +1,5 @@
 #include "test/jemalloc_test.h"
-#include "test/arena_decay.h"
+#include "test/arena_util.h"
 
 #include "jemalloc/internal/ticker.h"
 

test/unit/san.c

@@ -1,5 +1,5 @@
 #include "test/jemalloc_test.h"
-#include "test/arena_decay.h"
+#include "test/arena_util.h"
 #include "test/san.h"
 
 #include "jemalloc/internal/san.h"

test/unit/san_bump.c

@@ -1,5 +1,5 @@
 #include "test/jemalloc_test.h"
-#include "test/arena_decay.h"
+#include "test/arena_util.h"
 
 #include "jemalloc/internal/arena_structs.h"
 #include "jemalloc/internal/san_bump.h"