[lldb][test] Move registers-target-xml-reading target to the correct test location.

This test was added in D74217 (and the `.categories` file later added in ccf1c30c) around the same time I moved the test tree from `lldb/packages/Python/lldbsuite/test` to `lldb/test/API` (D71151). Since this got lost in the move, it isn't running. (I introduced an intentional syntax error, and `ninja check-lldb` passes).

I moved it to the correct location, and now it runs and passes -- locally, at least -- as `ninja check-lldb-api-tools-lldb-server-registers-target-xml-reading`.