tests/unit/test-char: implement a few mux remove test cases

tests/unit/test-char: implement a few mux remove test cases

This patch tests:

1. feasibility of removing mux which does not have frontends attached
   or frontends were prior detached.
2. inability to remove mux which has frontends attached (mux is "busy")

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
[ fixed *error leak ]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20241014152408.427700-9-r.peniaev@gmail.com>