test: compare DOM nodes by identity, never through assert.equal

A failed assert.equal or deepEqual prints both operands, and a happy-dom
node reaches the whole page and window: printing one exhausted memory and
took the machine down. assertSameNode() compares identity and fails with
a short description of each node; every node comparison in the happy-dom
tests uses it.

src/testSupport/domAssert.mjs

@@ -0,0 +1,19 @@
+import assert from 'node:assert/strict';
+
+const describe = (node) =>
+  node?.nodeType === 1
+    ? `<${node.tagName.toLowerCase()}${node.id ? `#${node.id}` : ''}>`
+    : String(node);
+
+/**
+ * Assert that two DOM nodes (or a node and null) are the same. Never hand
+ * happy-dom nodes to assert.equal or deepEqual: a failure prints both, and a
+ * node reaches the whole page and window, which runs the process out of
+ * memory.
+ */
+export function assertSameNode(actual, expected, message = '') {
+  assert.ok(
+    actual === expected,
+    `${message ? `${message}: ` : ''}${describe(actual)} is not ${describe(expected)}`,
+  );
+}