test(lua): cover vim._with() with tests

Problem: `vim._with()` has many different use cases which are not
  covered with tests.

Solution: cover with tests. Some (many) test cases are intentionally
  marked as "pending" because they cover cases which don't work as
  expected at the moment (and fixing them requires specific knowledge of
  C codebase). Use them as a reference for future fixes.
  Also some of "can be nested" tests currently might pass only because
  the tested context doesn't work.

runtime/lua/vim/shared.lua

@@ -1144,7 +1144,6 @@ end
 --- @field buf? integer
 --- @field emsg_silent? boolean
 --- @field hide? boolean
---- @field horizontal? boolean
 --- @field keepalt? boolean
 --- @field keepjumps? boolean
 --- @field keepmarks? boolean
@@ -1159,6 +1158,15 @@ end
 
 --- Executes function `f` with the given context specification.
 ---
+--- Notes:
+--- - Context `{ buf = buf }` has no guarantees about current window when
+---   inside context.
+--- - Context `{ buf = buf, win = win }` is yet not allowed, but this seems
+---   to be an implementation detail.
+--- - There should be no way to revert currently set `context.sandbox = true`
+---   (like with nested `vim._with()` calls). Otherwise it kind of breaks the
+---   whole purpose of sandbox execution.
+---
 --- @param context vim.context.mods
 function vim._with(context, f)
   vim.validate('context', context, 'table')
@@ -1167,7 +1175,6 @@ function vim._with(context, f)
   vim.validate('context.buf', context.buf, 'number', true)
   vim.validate('context.emsg_silent', context.emsg_silent, 'boolean', true)
   vim.validate('context.hide', context.hide, 'boolean', true)
-  vim.validate('context.horizontal', context.horizontal, 'boolean', true)
   vim.validate('context.keepalt', context.keepalt, 'boolean', true)
   vim.validate('context.keepjumps', context.keepjumps, 'boolean', true)
   vim.validate('context.keepmarks', context.keepmarks, 'boolean', true)
@@ -1192,6 +1199,10 @@ function vim._with(context, f)
     if not vim.api.nvim_win_is_valid(context.win) then
       error('Invalid window id: ' .. context.win)
     end
+    -- TODO: Maybe allow it?
+    if context.buf and vim.api.nvim_win_get_buf(context.win) ~= context.buf then
+      error('Can not set both `buf` and `win` context.')
+    end
   end
 
   -- Store original options
@@ -1214,7 +1225,7 @@ function vim._with(context, f)
     end
   end
 
-  return unpack(retval)
+  return unpack(retval, 1, table.maxn(retval))
 end
 
 return vim