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.

1 files changed, 6 insertions, 7 deletions

diff --git a/test/functional/core/fileio_spec.lua b/test/functional/core/fileio_spec.lua
index cf9715f848..b1a8e21762 100644
--- a/test/functional/core/fileio_spec.lua
+++ b/test/functional/core/fileio_spec.lua
@@ -31,7 +31,6 @@ local feed_command = n.feed_command
 local skip = t.skip
 local is_os = t.is_os
 local is_ci = t.is_ci
-local spawn = n.spawn
 local set_session = n.set_session
 
 describe('fileio', function()
@@ -51,12 +50,11 @@ describe('fileio', function()
     rmdir('Xtest_backupdir with spaces')
   end)
 
-  local args = { nvim_prog, '--clean', '--cmd', 'set nofsync directory=Xtest_startup_swapdir' }
+  local args = { '--clean', '--cmd', 'set nofsync directory=Xtest_startup_swapdir' }
   --- Starts a new nvim session and returns an attached screen.
-  local function startup(extra_args)
-    extra_args = extra_args or {}
-    local argv = vim.iter({ args, '--embed', extra_args }):flatten():totable()
-    local screen_nvim = spawn(argv)
+  local function startup()
+    local argv = vim.iter({ args, '--embed' }):flatten():totable()
+    local screen_nvim = n.new_session(false, { args = argv, merge = false })
     set_session(screen_nvim)
     local screen = Screen.new(70, 10)
     screen:set_default_attr_ids({
@@ -100,7 +98,8 @@ describe('fileio', function()
     eq('foozubbaz', trim(read_file('Xtest_startup_file1')))
 
     -- 4. Exit caused by deadly signal (+ 'swapfile').
-    local j = fn.jobstart(vim.iter({ args, '--embed' }):flatten():totable(), { rpc = true })
+    local j =
+      fn.jobstart(vim.iter({ nvim_prog, args, '--embed' }):flatten():totable(), { rpc = true })
     fn.rpcrequest(
       j,
       'nvim_exec2',