Regression test package (src/test/regress) is included in the distribution and let your computer do the verification for standard SQL operations as well as the extensibility capabilities of PostgreSQL. Advantage is that computer can rapidly run millions of SQL tests in a very short time. The speed of computer is about one billion times faster than human brain! In near future the speed of computer will be several zillion times faster than human brain! Hence, it makes sense to use the power of computer to validate the software instead of using the human brain power.
The test package already contains hundreds of SQL test programs. You can start the regression test (relax and have a cooldrink!) and come back take a glance at the output results and you are done! It is that simple and easy.
You can add more tests just in case you need to, and can upload to the primary PostgreSQL web site if you feel that it will be useful to others on internet. Regression test package helps build customer confidence and trust in PostgreSQL and facilitates rapid deployment of PostgreSQL on production systems without any major worries.
Regression test package can be taken as a "very solid" technical specification mutually agreed upon document between the developers and end-users. PostgreSQL developers extensively use the regression test package during development period and also before releasing the software to public to ensure good quality.