Why is my alias "NULL" in a test?
I tracked this down to the fact that current_path(), which returns path/to/content in the browser, instead returns batch when called from within Simpletest. 

Since drupal_lookup_path() uses the output of current_path() to generate the $cache[map] variable that it uses to find and store the path alias, this prevents drupal_lookup_path() from working in tests.

I have filed this as an issue on d.o.