runtime/trace: dump test traces on validation failure

runtime/trace: dump test traces on validation failure

We currently dump traces for internal/trace tests on validation failure,
but not for the runtime/trace package.

This change moves some of the machinery to do this into the testtrace
package and then uses it from the runtime/trace package.

For #75665.

Change-Id: Ibe2d4f3945c1fd21dcbccf56820865f8d2ea41f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/710755
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>