socat: add musl test

A couple of recent upgrades have broken socat on Musl, so it would be
nice to catch these in advance.

pkgs/tools/networking/socat/default.nix

@@ -5,6 +5,7 @@
 , readline
 , stdenv
 , which
+, buildPackages
 }:
 
 stdenv.mkDerivation rec {
@@ -30,6 +31,10 @@ stdenv.mkDerivation rec {
   nativeCheckInputs = [ which nettools ];
   doCheck = false; # fails a bunch, hangs
 
+  passthru.tests = lib.optionalAttrs stdenv.buildPlatform.isLinux {
+    musl = buildPackages.pkgsMusl.socat;
+  };
+
   meta = with lib; {
     description = "Utility for bidirectional data transfer between two independent data channels";
     homepage = "http://www.dest-unreach.org/socat/";