# Pastebin v5KSs9xQ
with import <nixpkgs> { };

let
  failures = lib.runTests {
    testAdd = {
      expr = 2 + 2;
      expected = 2;
    };
    testSub = {
      expr = 1 - 1;
      expected = 0;
    };
  };
in stdenv.mkDerivation {
  name = "test-results";
  runCommand =
    if builtins.length failures > 0 then "exit 1" else "echo 'OK' > $out";
}