refactor(tests): remove redir_exec #15718

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

test/helpers.lua

@@ -810,6 +810,6 @@ function module.read_nvim_log(logfile, ci_rename)
   return log
 end
 
-module = shared.tbl_extend('error', module, Paths, shared)
+module = shared.tbl_extend('error', module, Paths, shared, require('test.deprecated'))
 
 return module