test: define session_timeout in the host harness

test_port_tables and test_page_json link httpd/page_impl.c unmodified,
which now references session_timeout. Define it in support.c next to
the other firmware globals so the tests link again on current main.

test/support.c

@@ -27,6 +27,7 @@ uint8_t                  cmd_available;
 uint8_t                  err_status;
 uint8_t                  cmd_history[CMD_HISTORY_SIZE];
 uint16_t                 cmd_history_ptr;
+__xdata uint16_t         session_timeout = 200;   /* httpd global, referenced by page_impl.c */
 
 /* editor-internal read pointer (defined in cmd_editor.c, external linkage) */
 extern __xdata uint8_t l;