Override prepareTemplate

testCase.php

<?php

/**
 * Define constants after requires/includes
 * @param Text_Template $template
 * @return void
 */
public function prepareTemplate( Text_Template $template ) {
	$property = new ReflectionProperty( $template, 'template' );
	$property->setAccessible( true );
	$str = $property->getValue( $template );
	$str = str_replace( '{constants}', '', $str );
	$str .= "\n{constants}\n";
	$property->setValue( $template, $str );
	parent::prepareTemplate( $template );
}