Fix concurrency issues in Test_runWithTimeout

Use a concurrency-safe writer in runWithTimeout to avoid data races
on the bytes.Buffer passed in during tests.

Bug: 181095653
Fixes: 187149270
Test: Test_runWithTimeout
Test: go test -race ./cmd/run_with_timeout
Change-Id: I57a889765cb9ee7b42983f0906313e0c2d1e414e