internal/trace/tracev2: add test to validate spec invariants

internal/trace/tracev2: add test to validate spec invariants

This change adds a test to help guide people adding experiments to this
package by validating that the spec is written correctly.

Also, makes some minor tweaks to the package in order to get the tests
to pass.

Change-Id: I3daa420c5a9ec3ea536415c8e5d06f41666a9566
Reviewed-on: https://go-review.googlesource.com/c/go/+/646015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>