Test batches that return $context['finished'] > 1 do in fact complete. See http://drupal.org/node/600836
function testBatchLargePercentage() {
// Displaying the page triggers batch 5.
$this
->drupalGet('batch-test/large-percentage');
$this
->assertBatchMessages($this
->_resultMessages(1), t('Batch for step 2 performed successfully.'));
$this
->assertEqual(batch_test_stack(), $this
->_resultStack('batch_5'), t('Execution order was correct.'));
$this
->assertText('Redirection successful.', t('Redirection after batch execution is correct.'));
}