testers.nixosTest: Move warning and add link

This way ofborg doesn't see it and fail.
Unfortunately, it will cause more warnings than it otherwise
would, if multiple tests are evaluated in one go.

pkgs/build-support/testers/default.nix

@@ -118,7 +118,6 @@
        * make sense for this evaluation of Nixpkgs.
        */
       nixosTesting =
-        lib.warn "testers.nixosTest is deprecated. Use testers.runNixOSTest instead."
         (import ../../../nixos/lib/testing-python.nix {
           inherit (stdenv.hostPlatform) system;
           inherit pkgs;
@@ -136,6 +135,7 @@
             else test;
           calledTest = lib.toFunction loadedTest pkgs;
         in
+          lib.warn "testers.nixosTest is deprecated. Use testers.runNixOSTest instead. See https://nixos.org/manual/nixpkgs/unstable/#tester-runNixOSTest"
           nixosTesting.simpleTest calledTest;
 
   hasPkgConfigModule =