netns test: improve ping test

- Use fping for pinging multiple hosts in parallel.
  Significantly improves test runtime:
  >13 s -> ~200 ms for the negative ping tests.
- Only test network namespaces that are enabled.
  This allows running the netns test with a reduced service set for debugging.
- Remove deprecated services, instead add btcpayserver, spark-wallet

test/tests.nix

@@ -115,6 +115,7 @@ let testEnv = rec {
       nix-bitcoin.netns-isolation.enable = true;
       test.data.netns = config.nix-bitcoin.netns-isolation.netns;
       tests.netns-isolation = true;
+      environment.systemPackages = [ pkgs.fping ];
 
       # This test is rather slow and unaffected by netns settings
       tests.backups = mkForce false;