Tests setting messages and removing one before it is displayed.
function testSetRemoveMessages() {
// The page at system-test/drupal-set-message sets two messages and then
// removes the first before it is displayed.
$this
->drupalGet('system-test/drupal-set-message');
$this
->assertNoText('First message (removed).');
$this
->assertText('Second message (not removed).');
}