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

src/test/isolation/meson.build

@@ -58,7 +58,7 @@ isolationtester = executable('isolationtester',
 bin_targets += isolationtester
 
 tests += {
-  'name': 'main',
+  'name': 'isolation',
   'sd': meson.current_source_dir(),
   'bd': meson.current_build_dir(),
   'isolation': {

src/test/regress/meson.build

@@ -64,7 +64,7 @@ testprep_targets += refint_regress
 
 
 tests += {
-  'name': 'main',
+  'name': 'regress',
   'sd': meson.current_source_dir(),
   'bd': meson.current_build_dir(),
   'regress': {