And fix some test failures on the way.
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=54289
-- v2: ntdll: Do not use extended context in RtlCopyContext() if extended state is not copied. ntdll: Factor out context_copy_ranges() function. ntdll/tests: Fix random test failure in test_restore_context(). ntdll/tests: Fix xstate tests failing on modern Windows and CPUs with more xstate features. ntdll/tests: Fix the case of absent XSTATE context in test_copy_context().