Add timestamp to logfile

Add a second pipe to the setup_trace function which prepends a relative timestamp in seconds to the logs from autopkgtest. It can be hard to decipher logs without this as it's difficult to tell where a test is hanging and holding up a process when you're not monitoring it directly. The MR uses awk to prepend a timestamp in seconds since the beginning of the execution to the log messages in the log file (not in stdout when you're reading it from terminal)