[12.x] - add dispatchedOnce methods

[lioneaglesolutions]

When using the assertDispatchedTimes method, it can be confusing because the default parameter is 1.

Sometimes we want to know that something was dispatched exactly once and be able to easily read this at a glance.

Currently you could just do:

Event::assertDispatchTimes($myEvent, 1);

However some tools remove the 1 as it is "dead code" due to 1 being the default argument. So it becomes:

Event::assertDispatchTimes($myEvent);

This reads like an unfinished sentence.

This PR adds:

Event::assertDispatchedOnce(...)

This is very explicit and easy to read.

[taylorotwell]

Your methods should just call assertDispatchTimes internally so we don't duplicate all the code.

[lioneaglesolutions]

@taylorotwell done!