Move `textNodes` into DOM test utilities for reuse

src/annotator/anchoring/test/text-range-test.js

 import { TextPosition, TextRange, ResolveDirection } from '../text-range';
 
-import { assertNodesEqual } from '../../../test-util/compare-dom';
+import { assertNodesEqual, textNodes } from '../../../test-util/compare-dom';
 
 const html = `
 <main>
@@ -12,22 +12,6 @@ const html = `
 </main>
 `;
 
-/**
- * Return all the `Text` descendants of `node`
- *
- * @param {Node} node
- * @return {Text[]}
- */
-function textNodes(node) {
-  const nodes = [];
-  const iter = document.createNodeIterator(node, NodeFilter.SHOW_TEXT);
-  let current;
-  while ((current = iter.nextNode())) {
-    nodes.push(current);
-  }
-  return nodes;
-}
-
 describe('annotator/anchoring/text-range', () => {
   describe('TextPosition', () => {
     let container;

src/test-util/compare-dom.js

@@ -46,3 +46,19 @@ export function assertNodesEqual(actual, expected) {
     );
   }
 }
+
+/**
+ * Return all the `Text` descendants of `node`
+ *
+ * @param {Node} node
+ * @return {Text[]}
+ */
+export function textNodes(node) {
+  const nodes = [];
+  const iter = document.createNodeIterator(node, NodeFilter.SHOW_TEXT);
+  let current;
+  while ((current = iter.nextNode())) {
+    nodes.push(current);
+  }
+  return nodes;
+}