test: build dependency for event unit tests

The test_event_base and test_event_probe unit tests use function
event_register() which depends on CONFIG_EVENT_DYNAMIC=y.

Fixes: 7d02645f ("event: Add a simple test")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>