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/core/remote_spec.lua

@@ -10,10 +10,8 @@ local expect = n.expect
 local fn = n.fn
 local insert = n.insert
 local nvim_prog = n.nvim_prog
-local new_argv = n.new_argv
 local neq = t.neq
 local set_session = n.set_session
-local spawn = n.spawn
 local tmpname = t.tmpname
 local write_file = t.write_file
 
@@ -32,8 +30,7 @@ describe('Remote', function()
   describe('connect to server and', function()
     local server
     before_each(function()
-      server = spawn(new_argv(), true)
-      set_session(server)
+      server = n.clear()
     end)
 
     after_each(function()
@@ -49,7 +46,7 @@ describe('Remote', function()
       -- to wait for the remote instance to exit and calling jobwait blocks
       -- the event loop. If the server event loop is blocked, it can't process
       -- our incoming --remote calls.
-      local client_starter = spawn(new_argv(), false, nil, true)
+      local client_starter = n.new_session(true)
       set_session(client_starter)
       -- Call jobstart() and jobwait() in the same RPC request to reduce flakiness.
       eq(
@@ -144,15 +141,8 @@ describe('Remote', function()
 
   describe('exits with error on', function()
     local function run_and_check_exit_code(...)
-      local bogus_argv = new_argv(...)
-
-      -- Create an nvim instance just to run the remote-invoking nvim. We want
-      -- to wait for the remote instance to exit and calling jobwait blocks
-      -- the event loop. If the server event loop is blocked, it can't process
-      -- our incoming --remote calls.
-      clear()
-      -- Call jobstart() and jobwait() in the same RPC request to reduce flakiness.
-      eq({ 2 }, exec_lua([[return vim.fn.jobwait({ vim.fn.jobstart(...) })]], bogus_argv))
+      local p = n.spawn_wait { args = { ... } }
+      eq(2, p.status)
     end
     it('bogus subcommand', function()
       run_and_check_exit_code('--remote-bogus')