In normal usage, the io.Writer passed to the watcher is typically
`os.Stdout` and does not require locking because it is not being read
from inside the same process.
In tests however, the io.Writer was a *bytes.Buffer which is read
concurrently from another goroutine, introducing a race condition. This
fixes the issue in the test suite by introducing a wrapper around
*bytes.Buffer which implements the required locking.
cbdcef097f