electrum: grab tests from github, but use signed sources

Not crazy about this solution, but seems better than
not running tests or ignoring upstream's signatures.

pkgs/applications/misc/electrum/default.nix

@@ -1,6 +1,8 @@
-{ stdenv, fetchurl, python3, python3Packages, zbar, secp256k1 }:
+{ stdenv, fetchurl, fetchFromGitHub, python3, python3Packages, zbar, secp256k1 }:
 
 let
+  version = "3.3.5";
+
   qdarkstyle = python3Packages.buildPythonPackage rec {
     pname = "QDarkStyle";
     version = "2.5.4";
@@ -10,17 +12,35 @@ let
     };
     doCheck = false; # no tests
   };
+
+  # Not provided in official source releases, which are what upstream signs.
+  tests = fetchFromGitHub {
+    owner = "spesmilo";
+    repo = "electrum";
+    rev = version;
+    sha256 = "11rzzrv5xxqazcb7q1ig93d6cisqmd1x0jrgvfgzysbzvi51gg11";
+
+    extraPostFetch = ''
+      mv $out ./all
+      mv ./all/electrum/tests $out
+    '';
+  };
 in
 
 python3Packages.buildPythonApplication rec {
   pname = "electrum";
-  version = "3.3.5";
+  inherit version;
 
   src = fetchurl {
     url = "https://download.electrum.org/${version}/Electrum-${version}.tar.gz";
     sha256 = "1csj0n96zlajnrs39wsazfj5lmy7v7n77cdz56lr8nkmchh6k9z1";
   };
 
+  postUnpack = ''
+    # can't symlink, tests get confused
+    cp -ar ${tests} $sourceRoot/electrum/tests
+  '';
+
   propagatedBuildInputs = with python3Packages; [
     aiorpcx
     aiohttp