[FIX] web_editor: ensure standard selection before running tests

Some editor tests may be sensitive to the existence or not of a
selection in the DOM before initializing the editor. The first test to
be run won't have a selection before the editable is initialized, but
every subsequent test will (removing the test node add the end moves the
selection to its parent). One particularly problematic consequence
concerns tests that would fail with an initial selection but succeed
otherwise. Such a test would only fail when run after another test, but
not when run alone.

This commit prevents this confusing debugging situation (and potential
false negatives) by setting a selection at the beginning of the testNode
by default before running each test, ensuring that each test has the
same initial conditions.

Part-of: odoo/odoo#141303

addons/web_editor/static/src/js/editor/odoo-editor/test/utils.js

@@ -307,6 +307,12 @@ export async function testEditor(Editor = OdooEditor, spec, options = {}) {
     // the editor as otherwise those would genererate mutations the editor would
     // consider and the tests would make no sense.
     testNode.innerHTML = spec.contentBefore;
+    // Setting a selection in the DOM before initializing the editor to ensure
+    // every test is run with the same preconditions.
+    setTestSelection({
+        anchorNode: testNode.parentElement, anchorOffset: 0,
+        focusNode: testNode.parentElement, focusOffset: 0,
+    });
     const selection = parseTextualSelection(testNode);
 
     const editor = new Editor(testNode, Object.assign({ toSanitize: false }, options));