Do not use t() for the assert messages

Similarly to what done in Drupal core tests, the assert messages should not be translatable.

AccessDeniedTestCase

$this->assertText($node->title, 'Found the custom 403 page');
// Logout and check that the user login block is shown on custom 403 pages.
$this->drupalLogout();
$this->drupalGet('admin');
$this->assertText($node->title, 'Found the custom 403 page');

CacheIsEmptyCase

$this->assertTrue(cache_is_empty($this->default_bin), 'The cache bin is empty');

The last class code is not very consistent, since it translates the messages for its own assert methods.