Revert "Chore: remove doc for test filter"

This reverts commit 1e84710.

Author: gbudjeakp

docs/api/config/testFilter.md

@@ -0,0 +1,163 @@
+---
+layout: page-api
+title: QUnit.config.testFilter
+excerpt: Programmatically filter which tests to run.
+groups:
+  - config
+version_added: "3.0.0"
+---
+
+Programmatically filter which tests to run.
+
+<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 `testFilter` property allows you to implement custom logic for filtering which tests to run at runtime. This is useful for advanced scenarios such as:
+
+* Quarantining flaky tests in CI environments
+* Distributing tests across parallel workers
+* Dynamically skipping tests based on runtime capabilities
+* Loading filter criteria from external sources (APIs, files, etc.)
+
+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 hierarchy
+
+The `testFilter` callback runs as part of a layered filtering system:
+
+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)
+
+This layering enables:
+* Test authors to control test execution via `test.only()`, `test.skip()`, `test.if()`
+* Project maintainers to implement dynamic filtering via `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)
+* [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 quarantineList = ['flaky network test', 'timing-dependent test'];
+
+QUnit.config.testFilter = function (testInfo) {
+  if (process.env.CI === 'true') {
+    const isQuarantined = quarantineList.some(function (pattern) {
+      return testInfo.testName.indexOf(pattern) !== -1;
+    });
+    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;
+};
+```
+
+### Runtime capability detection
+
+Skip tests for features not available in the current environment.
+
+```js
+const features = {
+  webgl: typeof WebGLRenderingContext !== 'undefined',
+  indexedDB: typeof indexedDB !== 'undefined'
+};
+
+QUnit.config.testFilter = function (testInfo) {
+  // Tests tagged with [feature] in their name
+  for (const feature in features) {
+    const tag = '[' + feature + ']';
+    if (testInfo.testName.indexOf(tag) !== -1 && !features[feature]) {
+      console.log('[FEATURE] Skipping ' + feature + ' test');
+      return false;
+    }
+  }
+  return true;
+};
+
+QUnit.test('[webgl] 3D rendering', function (assert) {
+  // Only runs if WebGL is available
+  assert.ok(true);
+});
+```
+
+### Combine multiple conditions
+
+```js
+const quarantined = ['flaky test'];
+
+QUnit.config.testFilter = function (testInfo) {
+  // Skip quarantined tests
+  if (quarantined.some(function (p) { return testInfo.testName.indexOf(p) !== -1; })) {
+    return false;
+  }
+
+  // Skip slow tests in quick mode
+  if (process.env.QUICK_RUN && testInfo.testName.indexOf('slow') !== -1) {
+    return false;
+  }
+
+  // Filter by module if specified
+  if (process.env.TEST_MODULE && testInfo.module.indexOf(process.env.TEST_MODULE) === -1) {
+    return false;
+  }
+
+  return true;
+};
+```