GitHub Actions: per-test results from the JUnit reports, and shorter action references

[drowaudio]

Follow-up to #193.

Per-test results

The validate and acceptance actions wrote a JUnit report per plugin / config but only reported pass / fail from the exit code.

• common/junit.awk (POSIX awk, no dependencies) reads each report for the test count and the name and message of every failed test.
• Job summary: the table gains a "Tests" column (17, or 2 of 17 failed), followed by a "Failed tests" list grouped by plugin / config.
• Annotations: the one error annotation per failed plugin / config now lists its failed tests and messages. Still one per item, as GitHub caps error annotations at 10 per step.
• When there is no report (an array config, or a crash before one is written) the reason shown is the action's own message or the exit code.
• New report-dir input on both actions, so the reports can be written inside the workspace.
• README: where results appear, plus how to publish every test as its own PR check with a reporter action (needs checks: write, so not available to fork PRs).

Pass / fail is still decided by the exit code only.

Shorter references

The actions are now Tracktion/pluginval/validate@v2 and Tracktion/pluginval/acceptance@v2 (were Tracktion/pluginval/.github/actions/...). Only each action.yml moved to the repo root (validate/, acceptance/); the scripts, common/ and the README stay in .github/actions/ and are found relative to $GITHUB_ACTION_PATH. Nothing has been released against the old paths.

Testing

• All local cases (pass, wrong reference, array config, missing reference with and without recording, empty dir, no plugin, plugin glob, missing plugin, empty list) run under macOS bash 3.2 / BSD awk.
• The dogfood steps in acceptance.yml now use ./validate and ./acceptance, so they exercise the new locations and the parser on Linux, macOS and Windows. They don't assert on the summary text.
• The reporter-action example in the README hasn't been run.