Add Bamboo.TestAdapter.forward/2

[jonleighton]

This provides a way to receive emails from other processes without
resorting to shared mode and synchronous tests.

For background, this builds on some work that I did to allow Wallaby
tests to play nicely with Mox, while still allowing the tests to be
async. I wrote about it here:

https://jonleighton.name/2021/asynchronous-browser-tests-with-phoenix/

And then contributed some docs to Wallaby about the setup here:

elixir-wallaby/wallaby#592

This commit basically implements a kind of Mox.allow/3 function for
Bamboo.TestAdapter.

Note that I’ve changed the order of the arguments for this
Bamboo.TestAdapter.forward/2 function. For Mox.allow/3, the owner_pid
comes first:

Mox.allow(MyMock, owner_pid, child_pid)

But for Bamboo.TestAdapter.forward/2, the child_pid comes first:

Bamboo.TestAdapter.forward(child_pid, owner_pid)

My reasoning is that in the first example we’re allowing the child_pid
to access mocks defined by the owner pid. But in the second example,
we’re forwarding emails FROM the child_pid TO the owner_pid. So this
order of arguments seemed to make sense to me, but may be slightly
confusing.

[germsvel]

Thanks for opening this PR @jonleighton! I really like the idea of what you're doing here. I'll try to find some time later this week to give it a good review.