tests: poll: fix misconstructed k_poll_event test

Since commit 0c23cf94 ("include/kernel: check type of object passed
to K_POLL_EVENT_INITIALIZER") it is no longer possible to use arbitrary
type literals with the initializer macro. Misconstructing a k_poll_event
type for test purpose must be done explicitly.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>