Add tests to handle different exit conditions, and I/O patterns

[justin-stephenson]

Test improvements:

1. We should have tests which cover invoking tlog-rec-session in different ways, ensuring all I/O is transferred and the tlog program exits when the recorded child program exits - for example:

   $ tlog-rec-session -c ls
   $ whoami | tlog-rec-session -c ls
   $ ssh recuser@remotehost 'ls'
   $ echo "test" | ssh recuser@remotehost ls
   $ echo 'print("hello world")'| ssh recuser@remotehost '/bin/sh -c '"'"'/usr/bin/python3.7 && sleep 0'"'"

2. We need to test all I/O is transferred with a program that writes a high volume of output in a short time, and exits immediately.

3. Also, we need to add a test for the shut_and_wait.sh scenario described in https://github.com/Scribery/tlog/pull/276