Run test variants on different systems

[lengau]

We have many tests that have the same logic but need to be applied with slightly different data on a slightly different OS.

Rather than copying and pasting the task.yaml and having to update it in many places, it would be much nicer to be able to select the system(s) we run on based on the variant we're running.

[jonathan-conder]

You can do this already if the variants are defined at the project or backend level:

project: myproject

path: /path

environment:
  VAR/jammy: jammy
  VAR/noble1: noble1
  VAR/noble2: noble2

backends:
  lxd:
    systems:
      - ubuntu-22.04:
          variants: [jammy]
      - ubuntu-24.04:
          variants: [-jammy]

suites:
  tests/:
    summary: Summary

$ spread -list
lxd:ubuntu-22.04:tests/one:jammy
lxd:ubuntu-24.04:tests/one:noble1
lxd:ubuntu-24.04:tests/one:noble2