Enable theme
<?php
theme_enable(array('mytheme'));
variable_set('theme_default', 'mytheme');
and for the test
<?php
$this->drupalGet('/');
$this->assertRaw('themes/mytheme');
<?php
theme_enable(array('mytheme'));
variable_set('theme_default', 'mytheme');
and for the test
<?php
$this->drupalGet('/');
$this->assertRaw('themes/mytheme');