Docs: Add docs for QUnit.config.testFilter

Author: Krinkle

docs/api/config/testFilter.md

@@ -0,0 +1,130 @@
+---
+layout: page-api
+title: QUnit.config.testFilter
+excerpt: Callback to programmatically filter which tests to run.
+groups:
+  - config
+version_added: "2.25.0"
+---
+
+Callback to programmatically control which tests to run, and which tests to skip or filter out.
+
+<table>
+<tr>
+  <th>type</th>
+  <td markdown="span">`function` or `null`</td>
+</tr>
+<tr>
+  <th>default</th>
+  <td markdown="span">`null`</td>
+</tr>
+</table>
+
+The `QUnit.config.testFilter` callback allows you to implement a custom test filter at runtime. This is useful for advanced scenarios such as:
+
+* Distributing tests across parallel workers
+* Loading filter criteria from external sources (APIs, files, etc.)
+* Quarantining flaky tests in CI environments
+
+The callback receives a `testInfo` object and must return a boolean:
+* Return `true` to run the test
+* Return `false` to skip the test
+
+If the callback throws an error, the error is logged as a warning and the test is skipped.
+
+## Filter order
+
+Tests are filtered in a hierarchy of three layers:
+
+1. **Test-level filters** run first: [`test.only()`](../QUnit/test.only.md), [`test.skip()`](../QUnit/test.skip.md), [`test.if()`](../QUnit/test.if.md)
+2. **Programmatic filter** runs next: `QUnit.config.testFilter`
+3. **CLI/URL filters** run last: [`--filter`](./filter.md), [`--module`](./module.md), [`--moduleId`](./moduleId.md), [`--testId`](./testId.md)
+
+These target three different audiences or contexts:
+* Test authors to control test execution via `test.only()`, `test.skip()`, `test.if()`
+* Project maintainers to implement dynamic filtering via  `QUnit.config.testFilter`
+* Developers to manually filter tests via CLI or browser URL parameters
+
+## Parameters
+
+### `testInfo` (object)
+
+| Property | Type | Description |
+|----------|------|-------------|
+| `testId` | string | Internal hash identifier (used by "Rerun" links)
+| `testName` | string | Name of the test
+| `module` | string | Name of the parent module
+| `skip` | boolean | Whether test was already marked to skip
+
+## See also
+
+* [QUnit.config.filter](./filter.md) 
+* [QUnit.config.module](./module.md)
+* [Flat config](./index.md#flat-preconfig), including `qunit_config_filter` and `qunit_config_module` environment variables 
+* [QUnit.test.if()](../QUnit/test.if.md)
+
+## Examples
+
+### Quarantine flaky tests
+
+Use an external quarantine list to skip unstable tests in CI without modifying test code.
+
+```js
+const flakyDatabase = ['my network test', 'my timing-dependent test'];
+
+QUnit.config.testFilter = function (testInfo) {
+  if (process.env.CI === 'true') {
+    const isQuarantined = flakyDatabase.some(function (pattern) {
+      return testInfo.testName.includes(pattern);
+    });
+    if (isQuarantined) {
+      console.log('[QUARANTINE] Skipping: ' + testInfo.testName);
+      return false;
+    }
+  }
+  return true;
+};
+```
+
+### Parallel test sharding
+
+Distribute tests across multiple workers using deterministic hash-based assignment.
+
+```js
+const WORKER_ID = parseInt(process.env.WORKER_ID, 10);
+const TOTAL_WORKERS = parseInt(process.env.TOTAL_WORKERS, 10);
+
+QUnit.config.testFilter = function (testInfo) {
+  let hash = 0;
+  for (let i = 0; i < testInfo.testId.length; i++) {
+    const char = testInfo.testId.charCodeAt(i);
+    hash = ((hash << 5) - hash) + char;
+    hash = hash & hash;
+  }
+  hash = Math.abs(hash);
+
+  // Assign test to worker
+  const assignedWorker = hash % TOTAL_WORKERS;
+  return assignedWorker === WORKER_ID;
+};
+```
+
+### Combine multiple conditions
+
+```js
+const flakyDatabase = ['my network test', 'my timing-dependent test'];
+
+QUnit.config.testFilter = function (testInfo) {
+  // Skip quarantined tests
+  if (flakyDatabase.some((pattern) => testInfo.testName.includes(pattern))) {
+    return false;
+  }
+
+  // Skip slow tests in quick mode
+  if (process.env.QUICK_RUN && testInfo.testName.includes('slow')) {
+    return false;
+  }
+
+  return true;
+};
+```