Both ert-run-tests-interactively and ert-run-tests-batch
are implemented on top of the lower-level test handling code in the
sections of ert.el labeled “Facilities for running a single test”,
“Test selectors”, and “Facilities for running a whole set of tests”.
If you want to write code that works with ERT tests, you should take a
look at this lower-level code. Symbols that start with ert--
are internal to ERT, whereas those that start with ert- are
meant to be usable by other code. But there is no mature API yet.
Contributions to ERT are welcome.