integration-test: Fix occasional failure of test_luks_forced_removal

integration-test: Fix occasional failure of test_luks_forced_removal

Sometimes the client's GDBusObjectManager does not refresh properly and keeps
around objects which have long gone, which leads to a timeout in waiting for
the device object to disappear. Presumably this is because the test suite does
not run a main loop, and we cannot currently access
g_main_context_get_default() from introspection. So instead, just generate a
fresh client.