[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 ccf1c30cde) 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`.

lldb/test/API/tools/lldb-server/registers-target-xml-reading/TestGdbRemoteTargetXmlPacket.py

@@ -66,4 +66,4 @@ class TestGdbRemoteTargetXmlPacket(gdbremote_testcase.GdbRemoteTestCaseBase):
             self.assertEqual(q_info_reg["format"], xml_info_reg.get("format"))
             self.assertEqual(q_info_reg["bitsize"], xml_info_reg.get("bitsize"))
             self.assertEqual(q_info_reg["offset"], xml_info_reg.get("offset"))
-            self.assertEqual(q_info_reg["encoding"], xml_info_reg.get("encoding"))
+            self.assertEqual(q_info_reg["encoding"], xml_info_reg.get("encoding"))