test: use exec_capture() in more places (#22787)

Problem:
Using `meths.exec2("code", { output = true })` is too verbose.

Solution:
Use exec_capture() in more places.

test/functional/helpers.lua

@@ -826,7 +826,7 @@ function module.skip_fragile(pending_fn, cond)
 end
 
 function module.exec(code)
-  module.meths.exec2(code, { output = false })
+  module.meths.exec2(code, {})
 end
 
 function module.exec_capture(code)