Add testAddsMissingSelectors to ensure we add all selectors we need to
get events from iOS.

The change made to setOneSignalDelegate was required to make it
testable. This equality check is already done in setOneSignalUNDelegate
so this is also helping match our logic between these.

Added one failing test for the case where forwardingTargetForSelector
is not accounted for in swizzling. The most common case for this is a
SwiftUI app with @UIApplicationDelegateAdaptor.

Just added one test and fixed one case to keep this PR small and
readable, will add the rest in a follow up commit.

iOS 9 selectors we only swizzle on iOS 9 since
UIApplicationDelegateAdaptor requires a newer version of iOS.

This fixes a bug where an infinite loop happens when we call the origin
implementation if delegates we swizzle if something attempts to set the
delegate back to the original class. This would only happen if the class
didn't have the target selector to begin with when we run this swizzle
logic, but then it is called a 2nd time with same target class. We have
a check in the setOneSignalDelegate and setOneSignalUNDelegate but if
the app developer or another library changes the delegate then puts it
back this scenario can happen.

Added a test for this scenario, ensuring it fails and then passing
after. This also fixes an issue in the last commit where this bug was
causing tests after UIApplicationDelegateSwizzingTest to infinitely
loop.

This test broke in the last commit since the test was relying on
a 2nd call to injectSelector to unswizzle. This was intentionally done
as we don't want app or other SDKs swapping delegates back and forth to
cause OneSignal to no longer receive events.

This forced me to rewrite the OneSignalUNUserNotificationCenterHelper
used for tests to not reply on this behavior and in the process made
much simpler. It is declarative of the methods to swap back and
therefore much easier to understand.

Fix missing selector warns in this file from the last commit since.

Add test to ensure our swizzling still calls the original methods.