Instead of using the temporary `./testaliases` alias file, use a copy of
the `./testdata/aliases` file.

We're able to do this with our new ability to specify our alias file as
the first argument to our executable.

Use the TAP harness with `prove` to give ourselves an integration test
of the executable.

This checks that when passing an email to it over STDIN, the proper
alias gets added to the aliases file.

For the moment, we're using the temporary `./testaliases` file. In the
future we'll want to update our program and test code to use something
like `./testdata/tmp` or something like that.