Rename hooks module to test_hooks.

"Hooks" is really the best name for the module that will contain the publicly
exposed hooks.  So lets rename the current "hooks" module (that hook external
dependencies, for reentrancy testing) to "test_hooks".

src/arena.c

@@ -1900,8 +1900,8 @@ arena_new(tsdn_t *tsdn, unsigned ind, extent_hooks_t *extent_hooks) {
 		 */
 		assert(!tsdn_null(tsdn));
 		pre_reentrancy(tsdn_tsd(tsdn), arena);
-		if (hooks_arena_new_hook) {
-			hooks_arena_new_hook();
+		if (test_hooks_arena_new_hook) {
+			test_hooks_arena_new_hook();
 		}
 		post_reentrancy(tsdn_tsd(tsdn));
 	}

src/prof.c

@@ -23,7 +23,7 @@
  */
 #undef _Unwind_Backtrace
 #include <unwind.h>
-#define _Unwind_Backtrace JEMALLOC_HOOK(_Unwind_Backtrace, hooks_libc_hook)
+#define _Unwind_Backtrace JEMALLOC_HOOK(_Unwind_Backtrace, test_hooks_libc_hook)
 #endif
 
 /******************************************************************************/

src/test_hooks.c

@@ -6,7 +6,7 @@
  * from outside the generated library, so that we can use them in test code.
  */
 JEMALLOC_EXPORT
-void (*hooks_arena_new_hook)() = NULL;
+void (*test_hooks_arena_new_hook)() = NULL;
 
 JEMALLOC_EXPORT
-void (*hooks_libc_hook)() = NULL;
+void (*test_hooks_libc_hook)() = NULL;