test: soften traceback speed test

It's a bit flaky, we've been getting ratios up to the mid 7s.

We're getting close to the "this isn't testing anything" territory,
but if this issue really reverts the expected ratio is 10, so hopefully
the test will still fail at least fairly frequently.

Fixes #78136

Change-Id: Id45099a49d37f287c86fa4362bf6ac7ceaf00a43
Reviewed-on: https://go-review.googlesource.com/c/go/+/782900
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>