Fix WebVitals no interaction test

[inancgumus]

What?

Fixes the flaky test by waiting until the page that it goes to completely loads.

Why?

The test sometimes can't receive the WebVital metric emissions. This became evident thanks to #1018.

Failure example:

--- FAIL: TestWebVitalMetricNoInteraction (0.83s)
    webvital_test.go:124: 
                Error Trace:    /Users/inanc/grafana/k6browser/main/tests/webvital_test.go:124
                Error:          Should be true
                Test:           TestWebVitalMetricNoInteraction
                Messages:       expected browser_web_vital_cls to have been measured and emitted

Checklist

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas

[ankur22]

LGTM

[ka3de]

LGTM.