Tests: Replace use of `$this->server` with `rest_get_server()` in `test_registered_query_params()`.

In [42724], `$this->server` was replaced with `rest_get_server()` for better memory recycling.

[43897], from the 5.0 branch, was merged into trunk in [44250] and used the now unavailable `$this->server`.

This updates the new test from the 5.0 branch to use the expected `rest_get_server()`.

See #43316, #41641.
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git-svn-id: https://core.svn.wordpress.org/trunk@44085 1a063a9b-81f0-0310-95a4-ce76da25c4cd