internal/trace: add a validator test for the new clock snapshots

internal/trace: add a validator test for the new clock snapshots

Check that the clock snapshots, when expected to be present, are
non-zero and monotonically increasing.

This required some refactoring to make the validator aware of the
version of the trace it is validating.

Change-Id: I04c4dd10fe6975cbac12bb0ddaebcec3a5284e7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/669715
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>