accessibility-testing-js.md: Fix incorrect equality call

[illume]

My tests were failing with the way mentioned in the docs.

This was the error:

1) [chromium] › headlamp.spec.ts:38:5 › service page should have headlamp service ────────────────
    Error: expect(received).toBe(expected) // Object.is equality
    If it should pass with deep equality, replace "toBe" with "toStrictEqual"
    Expected: []
    Received: serializes to the same string
       at headlampPage.ts:18
      16 |       console.error(JSON.stringify(accessibilityResults.violations));
      17 |     }
    > 18 |     expect(accessibilityResults.violations).toBe([]);
         |                                             ^
      19 |   }
      20 |
      21 |   async authenticate(token?: string) {
        at HeadlampPage.a11y (/home/runner/work/headlamp/headlamp/e2e-tests/tests/headlampPage.ts:18:45)
        at /home/runner/work/headlamp/headlamp/e2e-tests/tests/headlamp.spec.ts:44:3

toStrictEqual() works, so I made changes to the docs to use that.

[dgozman]

@illume I don't see any toBe() in the guide, and this PR does not seem to correct any toBe() calls. What exactly was failing for you? I think toEqual() is perfectly fine.

[mxschmitt]

Closing as per above. Feel free to start with an issue if you think there is a bug in our docs. Thanks!