Wait until iframes have been loaded

tests/Browser/Mail/PreviewTest.php

@@ -50,6 +50,7 @@ class PreviewTest extends TestCase
             $browser->withinFrame('#messagecontframe', function ($browser) {
                 $browser->waitFor('img.contactphoto');
 
+                $browser->waitUntilMissing('.loading');
                 // Privacy warning
                 $browser->assertVisible('#remote-objects-message.alert-warning')
                     ->assertSeeIn('#remote-objects-message', 'To protect your privacy remote resources have been blocked.');

tests/Browser/Mail/PrintTest.php

@@ -55,6 +55,7 @@ class PrintTest extends TestCase
             });
 
             // Check iframed body.
+            $browser->waitUntilMissing('.loading');
             $browser->withinFrame('#message-part1 .framed-message-part', static function ($browser) {
                 $browser->assertSeeIn('div.pre', 'Plain text message body.')
                     ->assertVisible('div.pre .sig');