rustc: add passthru.tests

we expect these to be built as part of rust version bumps

pkgs/development/compilers/rust/rustc.nix

@@ -10,6 +10,9 @@
 , version
 , sha256
 , patches ? []
+, fd
+, firefox
+, thunderbird
 }:
 
 let
@@ -184,6 +187,7 @@ in stdenv.mkDerivation rec {
   passthru = {
     llvm = llvmShared;
     inherit llvmPackages;
+    tests = { inherit fd; } // lib.optionalAttrs stdenv.hostPlatform.isLinux { inherit firefox thunderbird; };
   };
 
   meta = with lib; {