Add tests for URL Repository

[abeyad]

The org.elasticsearch.repositories.uri.URLRepository class does not have any tests written for it. It needs tests to exercise its functionality, probably by connecting it to a webserver, so it will require some thought.

SharedClusterSnapshotRestoreIT contains two tests related to the URL repository:

1. testUrlRepository
2. testReadOnlyRepository

However, it would be helpful to have more comprehensive, unit level tests that prove the semantics and guarantees of the URL repository.

[nik9000]

We can use a fixture for this I think! See :test:fixtures.

[tlrx]

Closed by #29355