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/test_hooks.c

@@ -0,0 +1,12 @@
+#include "jemalloc/internal/jemalloc_preamble.h"
+
+/*
+ * The hooks are a little bit screwy -- they're not genuinely exported in the
+ * sense that we want them available to end-users, but we do want them visible
+ * from outside the generated library, so that we can use them in test code.
+ */
+JEMALLOC_EXPORT
+void (*test_hooks_arena_new_hook)() = NULL;
+
+JEMALLOC_EXPORT
+void (*test_hooks_libc_hook)() = NULL;