Test a successful upgrade (no negotiation).
public function testLocaleUpgrade() {
$this
->assertTrue($this
->performUpgrade(), 'The upgrade was completed successfully.');
// The home page should be in French.
$this
->assertPageInLanguage('', 'fr');
// No prefixed page should exist.
$this
->drupalGet('en');
$this
->assertResponse(404);
$this
->drupalGet('fr');
$this
->assertResponse(404);
}