meson: Split 'main' suite into 'regress' and 'isolation'
Several people didn't like the 'main' name and found it confusing that the
main regression and isolation tests were in one suite.
Author: Justin Pryzby <pryzbyj@telsasoft.com>
Discussion: https://postgr.es/m/
20221001221514.2yy257v4zdfhwiy2@awork3.anarazel.de
Discussion: https://postgr.es/m/
20221021123435.GU16921@telsasoft.com