fix: prevent Invalid URL error in prepareBaseUrl for http/https baseUrl

[yasinkocak]

What does this PR do?

Fixes an Invalid URL error that occurs when using baseUrl with http or https values in TestCafe 3.7.1+ on Node.js 20.18+.

Current behavior

The current implementation of prepareBaseUrl applies path.join to the provided baseUrl.
For web URLs, this produces invalid results:

• Input: "http://devexpress.github.io"
• Result after path.join: "http:/devexpress.github.io/"
• new URL("http:/devexpress.github.io/") → throws TypeError: Invalid URL

This breaks .page(...) and config-based baseUrl when using standard web URLs.

Expected behavior

• baseUrl is always treated as a web URL if it starts with http:// or https://.
• File system paths should still resolve correctly to file://.

How does this PR fix the problem?

• Detects and preserves http:// and https:// URLs (adds trailing slash if missing).
• Allows explicit file:// URLs.
• Falls back to converting absolute/relative file system paths via pathToFileURL.

Related issue

Fixes #8364

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