Fix file URL in test-static-stdlib

[jrflat]

URL(fileURLWithPath:) takes a URL path that doesn't include the file: scheme, so URL(fileURLWithPath: "file:///foo") treats the entire file:///foo as a relative path (since it doesn't start with a /).

In swiftlang/swift-foundation#792, we updated relative path parsing to no longer compress slashes (matching the behavior of all common RFC 3986 and WHATWG parsers), so the output of u.relativePath changed from file:/foo to file:///foo, causing this test to fail.

This PR updates the test to use the file URL API as expected and updates the check in accordance, e.g.

let u = URL(fileURLWithPath: "/foo")
print("foo bar baz: \(u.relativePath)")
// ...
CHECK: foo bar baz: /foo

and should fix the Linux builds.

[jrflat]

Linux smoke test passed at swiftlang/swift#75354

[jrflat]

All smoke tests passed

[etcwilde]

At least trying it on my mac, let u = URL(fileURLWithPath: "file:///foo") outputs file:/foo as the relative path. Should we continue to ensure that the outputs still match across implementations or does the root change expected to alter the behavior everywhere?

[jrflat]

At least trying it on my mac, let u = URL(fileURLWithPath: "file:///foo") outputs file:/foo as the relative path. Should we continue to ensure that the outputs still match across implementations or does the root change expected to alter the behavior everywhere?

We're working on updating the behavior to be the same across all Darwin and non-Darwin platforms, where not compressing the slashes will be expected behavior