The test instance of keystrokes does not implement the interface of browser binding sourced events

[RobertWHurst]

Originally reported by @noahbrom in #19

Describe the bug
If you use the keystrokes test instance, it will not implement the same interface for events that the browser bindings, which are used by Keystrokes by default. The typescript types also do not reflect this discrepancy.

To Reproduce
Write a unit test using a test instance of keystrokes, and attempt to use a browser binding sourced property on the event.

Expected behavior
The property should be the same as if used with the browser bindings (unless overridden).