Add browserContext.downloadsPath

[inancgumus]

What?

Users can set where to download files.

const context = await browser.newContext({
    acceptDownloads: true,
    downloadsPath: '/my/directory',
});

If downloadsPath is unset, we create a temporary directory for each BrowserContext with the k6-browser-artifacts- prefix. Note that we do this for every call to NewBrowserContext.

Why?

• Finding the downloaded files without downloadPaths can be difficult.
• Allow users to download files to a specific directory they want.
• To create a temporary directory to save the downloaded files.

Checklist

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

Related PR(s)/Issue(s)

Updates: #1289

[ankur22]

LGTM 🚀