tests: Add command source locations to test log

[roberth]

Motivation

Ctrl+click to the location of the test failure (heuristically in vscode)

Example:

    +(inputs.sh:39) writeSimpleFlake /tmp/nix-shell.VLOn9z/nix-test/tests/flakes/inputs/repo-32653/b-low
    +(./common.sh:13) local flakeDir=/tmp/nix-shell.VLOn9z/nix-test/tests/flakes/inputs/repo-32653/b-low
    +(./common.sh:14) cat
    +(./common.sh:34) cp ../simple.nix ../simple.builder.sh ../config.nix /tmp/nix-shell.VLOn9z/nix-test/tests/flakes/inputs/repo-32653/b-low/
    +(inputs.sh:41) echo all good
    +(inputs.sh:42) cat
    +(inputs.sh:55) cd /tmp/nix-shell.VLOn9z/nix-test/tests/flakes/inputs/repo-32653/b-low

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

[Ericson2314]

@roberth tells me for nested set -x-ing we in fact get ++++++(inputs.sh:55) and not a stack of filenames (which could be quite long). great!