Writing .spec files for PostgreSQL isolation tests

I am developing a PostgreSQL extension for which I would like to run some tests with concurrent connections. If I understand [the documentation](https://www.postgresql.org/docs/current/extend-pgxs.html) correctly, I should be able to do so using the PGXS framework that comes with Postgres. However, it is mentioned that such isolation tests (listed in the ISOLATION variable in my Makefile) have to be specified in a .spec file, and nowhere can I find a mention of how to write such a .spec file. I've failed to find an example in the documentation and I've similarly failed to find example code online. I know how to write the .sql tests that one can list in the REGRESS variable. Thus, my question is not about how to write tests in general. I specifically would like to know how to write a .spec file.