refactor(tests): merge n.spawn/n.spawn_argv into n.new_session #31859

Problem:
- `n.spawn()` is misleading because it also connects RPC, it's not just
  "spawning" a process.
- It's confusing that `n.spawn()` and `n.spawn_argv()` are separate.

Solution:
- Replace `n.spawn()`/`n.spawn_argv()` with a single function `n.new_session()`.
  This name aligns with the existing functions `n.set_session`/`n.get_session`.
    - Note: removes direct handling of `prepend_argv`, but I doubt that was
      important or intentional. If callers want to control use of `prepend_argv`
      then we should add a new flag to `test.session.Opts`.
- Move `keep` to first parameter of `n.new_session()`.
- Add a `merge` flag to `test.session.Opts`
- Mark `_new_argv()` as private. Test should use clear/new_session/spawn_wait
  instead.

test/functional/ex_cmds/wundo_spec.lua

@@ -5,8 +5,6 @@ local n = require('test.functional.testnvim')()
 local command = n.command
 local clear = n.clear
 local eval = n.eval
-local spawn = n.spawn
-local nvim_prog = n.nvim_prog
 local set_session = n.set_session
 
 describe(':wundo', function()
@@ -24,15 +22,11 @@ end)
 
 describe('u_* functions', function()
   it('safely fail on new, non-empty buffer', function()
-    local session = spawn({
-      nvim_prog,
-      '-u',
-      'NONE',
-      '-i',
-      'NONE',
-      '--embed',
-      '-c',
-      'set undodir=. undofile',
+    local session = n.new_session(false, {
+      args = {
+        '-c',
+        'set undodir=. undofile',
+      },
     })
     set_session(session)
     command('echo "True"') -- Should not error out due to crashed Neovim