Make `update_alias_id` tests more DRY

Eliminate some of the repeated setup code in our tests for this method.
Instead of doing this work inside the test functions, move them to a new
constant binding and function respectively.

A `const` seemed like a good fit because our alias identifier is just a
string. For the `Alias`, we have a new function that returns a
test object for us, making it possible to get two mutable copies of it
(one for each of our test functions).

Tests still pass, I guess it worked.

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