Fix test/unit/pages.

As part of the metadata_thp support, We now have a separate swtich
(JEMALLOC_HAVE_MADVISE_HUGE) for MADV_HUGEPAGE availability.  Use that instead
of JEMALLOC_THP (which doesn't guard pages_huge anymore) in tests.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -61,6 +61,13 @@ static const bool have_dss =
     false
 #endif
     ;
+static const bool have_madvise_huge =
+#ifdef JEMALLOC_HAVE_MADVISE_HUGE
+    true
+#else
+    false
+#endif
+    ;
 static const bool config_fill =
 #ifdef JEMALLOC_FILL
     true