Core: Add QUnit.config.testFilter to help filter tests

[gbudjeakp]

This PR implements QUnit.config.testFilter, a new configuration option that allows programmatic filtering of tests at runtime through a callback function.

Context:
Currently, QUnit provides filtering through CLI parameters (--filter, --module, etc.) and test-level methods test.only(), test.skip(), but there's no way to implement dynamic, programmatic filtering based on runtime conditions without modifying test code.

This feature enables several important use cases:

• Quarantining flaky tests in CI without modifying test code
• Parallel test execution by distributing tests across workers via hash-based sharding
• Runtime capability detection to skip tests for unavailable browser features (WebGL, IndexedDB, etc.)
• Dynamic test selection based on external data sources (APIs, configuration files)

[Krinkle]

Fixed macOS build in 9de4ec6.