Add utilities for testing with UUIDs

It's common to need to generate UUIDs for use within messages within tests, however when there is a test failure due to an incorrect UUID it can be hard to understand what the UUID refers to.

We could add a UUID generator to Test that obtains a UUID by "name", and logged about the UUIDs it generates in the test output. We could even annotate the diff output next to a UUID to show their "name".

I'm imagining usage something like the following:

test := testkit.Begin(t, app)
depositID := testkit.UUID("deposit ID")

test.Expect(
    testkit.ExecuteCommand(StartDeposit{
        TransactionID: depositID,
    }),
    assert.EventRecorded(DepositStarted{
        TransactionID: depositID,
    }),
)

This would result in log output something like:

Generated UUID for "deposit ID" (2a5c9fe5-29af-4f5b-a51d-8c306cfea9af)

No file or test is named. Start by locating the Test utilities and the existing testkit.Begin, testkit.ExecuteCommand, and test.Expect entry points, then determine where a testkit.UUID helper belongs. Done should include named UUID generation with the illustrated log output; UUID annotations in diff output are proposed but not specified.