taler-{exchange,merchant}: add passthru.tests

pkgs/by-name/ta/taler-exchange/package.nix

···
   gettext,
   texinfo,
   libtool,
+  nixosTests,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
···
   ];
 
   checkTarget = "check";
+
+  passthru.tests = nixosTests.taler.basic;
 
   meta = {
     description = "Exchange component for the GNU Taler electronic payment system";

pkgs/by-name/ta/taler-merchant/package.nix

···
   libgcrypt,
   texinfo,
   curl,
+  nixosTests,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
···
   nativeCheckInputs = [ jq ];
 
   checkTarget = "check";
+
+  passthru.tests = nixosTests.taler.basic;
 
   meta = {
     description = "Merchant component for the GNU Taler electronic payment system";