server/grab: add test case for ungrab during pointer-emulated grab
server/grab: add test case for ungrab during pointer-emulated grab

This currently triggers a memory corruption in the server, but requires
valgrind to reliably crash.

Related to bug #56578

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

freedesktop-unofficial-mirror/xorg__test__xorg-integration-tests,freedesktop-unofficial-mirror/xorg__test__xorg-integration-tests,freedesktop-unofficial-mirror/xorg__test__xorg-integration-tests