MenuXmlTest - Improve debug output. Use richer helpers.

MenuXmlTest - Improve debug output. Use richer helpers.

In automated test-runs, the e2e test for `mixin/menu-xml@1` (`MenuXmlTest`) has been failing sporadically.
The failure always involves one of the HTTP sub-requests, but it's never clear what actually happened
in the HTTP sub-request. This patch improves debug info.

Before
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Send HTTP sub-request with simple primitives (`cv` + `file_get_contents`).  No detailed information about the failed request.

After
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Send HTTP sub-request with `HttpTestTrait` (`guzzle`). Use rich assertions that log more detailed information.