refactor(tests): introduce testprg()

Also:
- Add a describe('shell :!') section to system_spec.
- Make the test for #16271 work on systems without powershell.

test/functional/terminal/helpers.lua

@@ -3,7 +3,7 @@
 -- operate on the _host_ session, _not_ the child session.
 local helpers = require('test.functional.helpers')(nil)
 local Screen = require('test.functional.ui.screen')
-local nvim_dir = helpers.nvim_dir
+local testprg = helpers.testprg
 local feed_command, nvim = helpers.feed_command, helpers.nvim
 
 local function feed_data(data)
@@ -37,7 +37,7 @@ local function clear_attrs() feed_termcode('[0;10m') end
 local function enable_mouse() feed_termcode('[?1002h') end
 local function disable_mouse() feed_termcode('[?1002l') end
 
-local default_command = '["'..nvim_dir..'/tty-test'..'"]'
+local default_command = '["'..testprg('tty-test')..'"]'
 
 local function screen_setup(extra_rows, command, cols, opts)
   extra_rows = extra_rows and extra_rows or 0