GitHub Actions: document DISABLED_TESTS and count the dogfood plugins

[drowaudio]

Follow-up to #191.

What

• README: how to skip tests with the validate action. Test names contain spaces, so they can't go in the whitespace-split args input; DISABLED_TESTS on the step's env works (comma-separated names, or an absolute path to a file listing them). Uses "Discrete parameter state restoration" as the example, since Add a discrete parameter state restoration test and fix false failures in the existing one #191 makes it fail at strictness 8+ for plugins using juce::AudioParameterBool, and the README's own example is at strictness 10.
• acceptance.yml: the fixtures step now counts the dogfood plugin bundles and passes both the glob and the count on, so the validate check compares against the actual number instead of a hard-coded 4. Adding a test plugin no longer needs a workflow edit. The check also fails if no plugins are found.

Testing

• Ran the validate action script locally with DISABLED_TESTS="Plugin state,Audio processing": the report dropped from 17 to 15 test cases, with both named tests gone.
• Ran the counting snippet locally under bash 3.2 against an existing build: bundle names containing spaces are counted once each.
• The workflow change is exercised by this PR's "Acceptance tests" run on all three platforms.