feat: add discover-path key to integration-suites to decouple suite identity from test directory

[javierdelapuente]

Problem

The suite key in integration-suites currently serves two purposes:

1. The unique suite identifier (used as the spread suite name and as the build/<key>/run/task.yaml path)
2. The filesystem path used for test auto-discovery

This makes it impossible to run the same set of tests twice with different environments (e.g. juju 3.6 and juju 4) within a single integration-suites block, since YAML keys must be unique.

Proposed solution

Add an optional discover-path: key to integration-suites entries. When set, opcli uses it for test discovery instead of the suite key. The suite key remains just a unique identifier (and a valid filesystem path for build/<key>/run/task.yaml generation).

Example

integration-suites:
  tests/integration/:
    cwd: ./
    summary: integration tests on juju 3.6
    backends:
    - integration-test

  tests/integration/juju4/:
    discover-path: tests/integration/
    discover-pattern: "*_jubilant.py"
    cwd: ./
    summary: jubilant tests on juju 4
    backends:
    - integration-test
    environment:
      CONCIERGE: concierge-juju4.yaml

Here tests/integration/juju4/ does not need to exist on disk — it's just a unique suite name. opcli discovers tests from discover-path instead.

Context

Encountered while adding juju 4 variants for jubilant integration tests in traefik-k8s-operator. The current workaround is to use manual MODULE/ variants, which partially works but loses the benefit of auto-discovery and requires keeping variant names in sync with test files.