OurBigBook Library tested on PostgreSQL

After something broke on the website due to SQLite vs PostgreSQL inconsistencies and took me a day to figure it out, I finally decided to update the test system so that OURBIGBOOK_POSTGRES=true npm test will run the tests on PostgreSQL.

Originally, these were being run only on SQLite, which is the major use case for OurBigBook CLI, which came before the website.

But the website runs on PostgreSQL, so it is fundamental to test things in PostgreSQL as well.