`expect().toContain()` error message doesn't show expected string

[marvinhagemeister]

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Describe the bug

When an expect().toContain() assertion failed we get a very generic error message:

error: AssertionError: The value doesn't contain the expected item
      throw new AssertionError("The value doesn't contain the expected item");

This makes it difficult to know what exactly went wrong.

Steps to Reproduce

import { expect } from "jsr:@std/expect";

Deno.test("foo", () => {
  expect("foobar").toContain("baz");
});

Expected behavior

Print something like Input string did not contain "baz". Not sure what to do for long strings though.

Environment

• OS: macOS 14.4.1
• deno version: 1.42.4
• std version: 0.224.0