doc/coding-conventions: Increase passthru test timeout 3s -> 60s

[roberth]

3s is too small a margin for a loaded slow system to start a bloated
program.
This leads to problems when tests are written on decent dev hardware
but later run on build farms of potentially slower hardware,
particularly in the case of non x86.

The chance of needing the timeout is actually very small, so those
rare 57s are a reasonable price to pay for fewer timeouts on build
farms, each of which should be investigated and usually fixed by
increasing the timeout.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Relase notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[roberth]

Refs #130204 (comment), #121896 (comment)

[davidak]

that's reasonable. thanks