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- `redir_exec` is obsolete, but it keeps getting used in new tests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/deprecated.lua

@@ -0,0 +1,9 @@
+-- Island of Misfit Toys
+
+local M = {}
+
+function M.redir_exec()
+  error('redir_exec is deprecated, use nvim_exec() or pcall_err()')
+end
+
+return M