Tests: Replace uses of `$this->server` with `rest_get_server()`.

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

[43862], from the 5.0 branch, was merged into trunk in [44225] and used the now unavailable `$this->server`.

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

See #45269, #41641.

Built from https://develop.svn.wordpress.org/trunk@44234


git-svn-id: https://core.svn.wordpress.org/trunk@44064 1a063a9b-81f0-0310-95a4-ce76da25c4cd