services/application/application: fix race condition in unit test
services/application/application: fix race condition in unit test

bytes.Buffer is not thread-safe. If two writes happen simultaneously,
data may get messed up. Instead, use a channel.

Change-Id: I2c4aaba40ca6c9276a30cd67f8dfaf61132e400e

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref