jenkins: skip SmartOS and AIX for pull request test runs

[codebytere]

Implements the proposal in #4457 in the one place this repo controls, VersionSelectorScript.groovy.

node-test-pull-request passes GIT_REMOTE_REF=refs/pull/N/head down through node-test-commit to every sub-job, while node-daily-*, node-release-candidate and the other upstream triggers pass a branch ref (checked against the parameters of recent node-test-commit-smartos builds from both). This adds a prTestType predicate keyed on that and excludes the smartos* and aix7* labels for PR-triggered test runs only. node-test-commit-smartos and -aix are still triggered by the multijob but select no configurations, which the matrix strategy reports as SUCCESS within seconds (BaseMES.run starts from SUCCESS and only folds in configuration results); daily, staging-daily and release runs are unchanged.

Dry-ran the script with emulated parameters / combinations bindings: a v27 PR run keeps rhel8-x64, macos15-x64, rhel9-s390x and drops smartos23-x64, aix72-power9, aix73-power9; the same labels with refs/heads/main, with no ref, or with buildType = 'release' keep everything. What i couldn't check from outside is whether either job has a parent-level post-build step that objects to zero configurations (a JUnit publisher, say); if so that wants "do not fail on empty results" alongside this.

Refs: #4457

---

Disclosure: the code and this description were written by Claude Code, directed and reviewed by @codebytere.

[richardlau]

I object to removing AIX from pull request test jobs (since we release binaries for it), but putting that to one side, this isn't the best way to exclude platforms from PR CIs as the parent jobs will still be run (this only exclude the matrix axes).

For e.g. IBM i (which is not run on PRs), we do not include the job in the node-test-pull-request Multijob and instead include it in node-daily-master.