fix: parse relative URLs with multiple leading slashes for special schemes

[ZachDreamZ]

What's wrong

When a relative URL starts with more than two slashes (e.g. ///test) and the base URL has a special scheme (http, https, ws, wss, ftp), the parser throws an error instead of correctly parsing it.

Url::options().base_url(Some(&Url::parse("http://example.org/").unwrap()))
    .parse("///test")
    .unwrap()
// Expected: http://test/
// Actual: Err(EmptyHost)

This matches Chrome, Firefox, Node, and Bun behavior. The URL spec's special authority slashes state says all leading slashes should be consumed before parsing the authority.

What this does

In parse_relative, when we have a base URL with a special scheme, we now route directly to after_double_slash with the remaining input, which correctly handles multiple leading slashes. The existing split_prefix("//") path is kept for non-special schemes.

Also removed 7 entries from expected_failures.txt that now pass correctly:

• ///test against http://example.org/
• ///example.org/path against http://example.org/
• and 5 more variants

Test plan

• All existing WPT tests should pass
• The 7 removed expected failures should now be correct
• Verified against the WPT test data for scheme-relative paths with multiple slashes

Context

This was previously reported as denoland/deno#35172 where new URL("///test", "http://example.org") throws in Deno but works in all other browsers and runtimes.

[mrobinson]

Duplicate of #1136.