internal/trace: run children in race mode during race testing

internal/trace: run children in race mode during race testing

This would have caught #65607.

On my machine:

internal/trace/v2:               ~13s
internal/trace/v2: -short:        ~4s
internal/trace/v2: -race -short: ~20s
internal/trace/v2: -race:        ~46s

For #65607.

Change-Id: I03a555ebaf8c761c98a82b6d35fa16d7a99aa9e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/562998
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>