Function assertEq

example/tests/gm_xhr_test.js:1944–1946

(a, b, msg)

    if (!condition) throw new Error(msg || "assertion failed");
  }
  function assertEq(a, b, msg) {
    if (a !== b) throw new Error(msg ? `${msg}: expected ${b}, got ${a}` : `expected ${b}, got ${a}`);
  }
  function assertDeepEq(a, b, msg) {
    const aj = JSON.stringify(a);
    const bj = JSON.stringify(b);