The mocks used in the external provider unit tests have been rewritten
to allow sending and inspecting messages in either direction. They also
no longer rely upon mocking out internal methods.

The `getInitializedInpageProvider` helper function in the inpage
provider tests has been updated with method callbacks, to allow tests
to setup replies for specific messages. This will be used in a future
PR.

One additional test was added to ensure the `isMetaMask` property is
set on the inpage provider.

The initialization step of the mock provider has been simplified. The
initialization event is awaited directly rather than using the more
complicated decomposed promise pattern.

The return type of `getInitializedProvider` is now an object rather
than a tuple. This was done to improve readability. Now it's more clear
exactly what each property is without close inspection.

The return type has been improved as well. Beforehand it was inferred
by TypeScript, but now it is explicitly declared as a type and
documented.

A protection measure was setup in the initialized provider helper
function to prevent it from responding to messages before the
initialization was complete This has been removed.

It was originally added during debugging, to rule out before-
initialization messages as an explanation for a bug that was later
fixed I left it in because I thought that it might better match
expectations if messages before initialization were ignored.

But in retrospect, I think this would be both unnecessary and harmful.
It's unnecessary because for most tests, no messages would be sent
until after initialization anyway. And it's potentially harmful because
this doesn't represent how the provider actually behaves. This could
disguise a mistake in a test, or it could get in the way of testing
strange edge cases surrounding initialization.