Add tests covering more cases for Uri.resolve

[stof]

As part of the scssphp project (which is a PHP port of dart-sass), I had to implement the Uri.resolve API of Dart in PHP (as this API does not implement only the RFC3986 resolution but more cases). My initial work used a testsuite ported from the Dart SDK tests but collecting code coverage on my PHP code revealed that those tests were far from covering all non-RFC3986 cases. It missed several cases:

• schemeless base URIs (for which the resolution is derived from the RFC3986 behavior)
• base URIs with a scheme but not authority
• base URIs with an empty path
• base URIs being an absolute path
• base URIs with only a query string
• base URIs being a relative path, resolving a reference that is not a relative path

This is adding tests covering all those cases.

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