testing, testing/synctest: write bubble errors to parent test log

testing, testing/synctest: write bubble errors to parent test log

Ensure that log messages written to the testing.T created by
synctest.Test appear in the test output when a test fails.

Fixes #73902

Change-Id: Ie97f5efe54eb003e6c0a5394c2def4cac1520ecb
Reviewed-on: https://go-review.googlesource.com/c/go/+/676995
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>