JS: add test for DOM access where expression appears to have no side effects

Author: Napalys

javascript/ql/test/query-tests/Expressions/ExprHasNoEffect/ExprHasNoEffect.expected

@@ -1,3 +1,5 @@
+| dom.js:5:3:5:18 | box.offsetHeight | This expression has no effect. |
+| dom.js:11:5:11:40 | documen ... ntWidth | This expression has no effect. |
 | try.js:22:9:22:26 | x.ordinaryProperty | This expression has no effect. |
 | tst2.js:2:4:2:4 | 0 | This expression has no effect. |
 | tst.js:3:1:3:2 | 23 | This expression has no effect. |

javascript/ql/test/query-tests/Expressions/ExprHasNoEffect/dom.js

@@ -0,0 +1,13 @@
+function f() {
+  const box = document.getElementById('box');
+  box.classList.remove('animate');
+  // Force a reflow by reading layout property (side effect!)
+  box.offsetHeight; // $SPURIOUS:Alert
+
+  try {
+    console.log("Hello, world!");
+  } catch (e) {
+    // Reading this property also forces layout
+    document.documentElement.clientWidth; // $SPURIOUS:Alert
+  }
+}