Motivation for this change

The login test for 16.09 has been failing lately, for no good reason. It appears that the typing for the username alice is too soon, causing this failure. This is hard to test, since it depends on the system being under load.

I added test primitives to access the raw TTY memory and see what is on the TTY. This could possibly be used to stabilize other tests, too.

Here is an example of where the retry helped prevent a race condition:

machine: sending keys ‘alice
’
machine: sending monitor command: sendkey a
machine: sending monitor command: sendkey l
machine: sending monitor command: sendkey i
machine: sending monitor command: sendkey c
machine: sending monitor command: sendkey e
machine: sending monitor command: sendkey ret
machine: waiting for login: alice to appear on tty 2
machine: running command: fold -w 80 /dev/vcs2
machine: exit status 0
machine: running command: fold -w 80 /dev/vcs2
machine: exit status 0

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.