[FIX] web: test broken in Firefox

This is arguably a 20 years old Firefox bug (at the very least an
under-specified area of the spec where Firefox's behaviour is
technically allowed under spec but not super useful or convenient):
`window.getSelection()` simply doesn't work when invoked on a form
field: https://bugzilla.mozilla.org/show_bug.cgi?id=85686.

Getting the selection data more explicitly by looking up the focused
form element, then checking *its* selection, seems to work fine and be
cross-browser.

Also focus() the input while at it: according to MDN

> Calling element.select() will not necessarily focus the input, so it
> is often used with HTMLOrForeignElement.focus.

And jQuery doesn't really document whether `select()` will implicitly
`focus()`, so better safe than sorry.

closes odoo/odoo#69579

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

addons/web/static/src/js/fields/basic_fields.js

@@ -881,6 +881,7 @@ var FieldDate = InputField.extend({
      */
     activate: function () {
         if (this.isFocusable() && this.datewidget) {
+            this.datewidget.$input.focus();
             this.datewidget.$input.select();
             return true;
         }

addons/web/static/tests/fields/basic_fields_tests.js

@@ -4314,7 +4314,7 @@ QUnit.module('basic_fields', {
     });
 
     QUnit.test('field date should select its content onclick when there is one', async function (assert) {
-        assert.expect(2);
+        assert.expect(3);
         var done = assert.async();
 
         var form = await createView({
@@ -4332,7 +4332,10 @@ QUnit.module('basic_fields', {
             'show.datetimepicker': function () {
                 assert.ok($('.bootstrap-datetimepicker-widget').is(':visible'),
                     'bootstrap-datetimepicker is visible');
-                assert.strictEqual(window.getSelection().toString(), "02/03/2017",
+                const active = document.activeElement;
+                assert.equal(active.tagName, 'INPUT', "The datepicker input should be focused");
+                const sel = active.value.slice(active.selectionStart, active.selectionEnd);
+                assert.strictEqual(sel, "02/03/2017",
                     'The whole input of the date field should have been selected');
                 done();
             }