Add utilities for testing with UUIDs

[jmalloc]

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)

[jmalloc]

We could even annotate the diff output next to a UUID to show their "name".

Perhaps this could be a more generic "annotation" system. It's probably worth looking into existing test-vector generation libraries too.

[jmalloc]

Low-level support for annotations is being added to dogmatiq/dapper in dogmatiq/dapper#68, so if we could provide a facade in testkit that sets up annotations when rendering message diffs that should get the job done. I don't think we actually need anything UUID specific, so now I'm thinking of an interface something like:

depositID := uuid.New() // regular UUID library
test := testkit.Begin(t, app)
test.Annotate(depositID, "the deposit ID")
test.Expect(...)

We'd have to come up with some rules for equality, and they'd have to work even if a value was serialized/deserialized again (that is, probably can't be a shallow comparison).