Literally copying a line from the source to be tested is perhaps not the best way to test that code. 😉
That said, we do want to verify that the function was actually loaded.
TODO: actually test the function.
For testing non-core files, just `load ../test_helper_libs` after `load ../test_helper` instead of trying to guess which libs must be loaded, in which order, for testing to succeed.
`_command_exists` spews log messages, so our test log is littered with `_log_debug not found`. This fixes that too.