Motivation for this change

This is done in two ways:

• Cherry-picks a particular issue (connecting to guest) and time how long it takes.
• Time every nest in the logs

The first timing is to track a specific event which might be the cause of our spurious failures in tests. That is, the alarm 300 might be too conservative on the build servers. Without data we cannot know.

The second part is to help track down issues in tests, hopefully tracking down whatever increase in time happened. The time was added as a log line at the end of the nest since there seems to be no way (with the current implementation) to add it as an attribute to the nest tag, since the XML writer thing seems to be made that way; we would have to defer writing the log until the nesting was closed. This should be generally machine-parseable, but I'm not entirely confident whether it should be implemented in another way to allow machine parsing.

Things done

• ✔️ Tested using sandboxing []((nix.useSandbox) on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • ✔️ NixOS
  • 🔲 macOS
  • 🔲 other Linux distributions
• ✔️ Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• 🔲 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/)
• 🔲 Determined the impact on package closure size (by running nix path-info -S before and after)
• ✔️ Assured whether relevant documentation is up to date
• ✔️ Fits CONTRIBUTING.md.