Run the Tests workflow for pull requests into any base branch #1321

Problem

.github/workflows/test.yml runs for pull_request events only when the pull request's base branch is main:

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

A stacked pull request targets another pull request's branch rather than main. GitHub therefore starts no Tests run for it: lint, engine-free, the country engine jobs, integration-uk and wheels are all skipped, and the pull request shows no checks at all.

We want to stack microcosm#1295, #1297, #1296 and #1298, which build on each other's changes. Without this change, only the bottom pull request of the stack would get CI.

Proposed change

• Remove the branches filter from the pull_request trigger, so a pull request into any base branch runs the same jobs.
• Leave the push trigger at branches: [main].
• Add a test that reads the workflow and checks both triggers.

What else in the workflow depends on the base branch

Nothing:

• the concurrency group is keyed on the pull request number;
• country selection (tools/ci_test_plan.py select-countries) reads the pull request's own changed files from the GitHub API;
• the workflow never reads github.base_ref;
• it has no path filters.

Pull requests from branches in this repository get the same GITHUB_TOKEN permissions and the same secrets whatever their base branch. Pull requests from forks get no secrets either way.