xapian: skip time-sensitive tests

I think I've seen this break builds on Hydra repeatedly, e.g:
https://hydra.nixos.org/build/123513191

pkgs/development/libraries/xapian/default.nix

@@ -18,6 +18,7 @@ let
     nativeBuildInputs = [ autoreconfHook ];
 
     doCheck = true;
+    AUTOMATED_TESTING = true; # https://trac.xapian.org/changeset/8be35f5e1/git
 
     patches = stdenv.lib.optionals stdenv.isDarwin [ ./skip-flaky-darwin-test.patch ];