notification tests [nfc]: Add Flow types.
notification tests [nfc]: Add Flow types.

Add type information to tests for notification-handling code.

`extractNotificationData` is fundamentally ill-typed as it currently
exists. It's on the chopping block, though, so the surfeit of
`$FlowFixMe`s involved there are of little concern.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile