move setup scripts to be nested inside scripts (#2379)

Much of this has been adapted from #2018, with attention paid towards
short-term back compat ("scripts.setup" instead of "script.setup").

Author: sunshowers

fixtures/nextest-tests/.config/nextest.toml

@@ -118,8 +118,8 @@ max-threads = 20
 [test-groups.serial]
 max-threads = 1
 
-[script.my-script-unix]
+[scripts.setup.my-script-unix]
 command = './scripts/my-script.sh'
 
-[script.my-script-windows]
+[scripts.setup.my-script-windows]
 command = 'cmd /c "scripts\\my-script.bat"'