internal/trace/internal/testgen: force trace version selection

internal/trace/internal/testgen: force trace version selection

Currently testgen only generates Go 1.22 tests. Allow generating tests
for different versions, especially now that we've tightened up which
events can be emitted by different versions.

Change-Id: Ia64309c6934f34eace03b3229d05fca5acfc7366
Reviewed-on: https://go-review.googlesource.com/c/go/+/644220
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>