[FIX] mail:tests: more precise jQuery selector on modal body content

Tests from [1] were not passing, due to changes from [2].

This is due to the fact that `$.('.modal-body')` picks 2 elements, instead of 1.

Partial HTML before [2]:

```
    <main class="modal-body">
        <div>
            <p>
                ...
            </p>
        </div>
    </main>
```

Partial HTML after [1]:

```
    <main class="modal-body">
        <main role="alert" class="modal-body">
            <p>
                ...
            </p>
        </main>
    </main>
```

In order to not pick both DOM elements, we refine the jQuery selector to pick
the `<p>`, so that there's only one element and it has the same text content.

[1] [FIX] mail: warn user on record reload before post message
    https://github.com/odoo/odoo/commit/b74d1e90ef32893bfbcf750cdf4586c453790b01
[2] [IMP] Improve compatibility with screen readers (accessibility) (#24574)
    https://github.com/odoo/odoo/commit/9de1bc0eef6f5bfaa2a8d745431caa361ae91548

addons/mail/static/tests/chatter_tests.js

@@ -654,7 +654,7 @@ QUnit.test('chatter: discard changes on message post with post_refresh "always"'
 
     var $modal = $('.modal-dialog');
     assert.strictEqual($modal.length, 1, "should have a modal opened");
-    assert.strictEqual($modal.find('.modal-body').text(),
+    assert.strictEqual($modal.find('.modal-body p').text(),
         "The record has been modified, your changes will be discarded. Do you want to proceed?",
         "should warn the user that any unsaved changes will be lost");
 
@@ -830,7 +830,7 @@ QUnit.test('chatter: discard changes on message post with post_refresh "recipien
 
             var $modal = $('.modal-dialog');
             assert.strictEqual($modal.length, 1, "should have a modal opened");
-            assert.strictEqual($modal.find('.modal-body').text(),
+            assert.strictEqual($modal.find('.modal-body p').text(),
                 "The record has been modified, your changes will be discarded. Do you want to proceed?",
                 "should warn the user that any unsaved changes will be lost");
 
@@ -883,7 +883,7 @@ QUnit.test('chatter: discard changes on opening full-composer', function (assert
 
     var $modal = $('.modal-dialog');
     assert.strictEqual($modal.length, 1, "should have a modal opened");
-    assert.strictEqual($modal.find('.modal-body').text(),
+    assert.strictEqual($modal.find('.modal-body p').text(),
         "The record has been modified, your changes will be discarded. Do you want to proceed?",
         "should warn the user that any unsaved changes will be lost");