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/ui/hlstate_spec.lua

@@ -5,7 +5,7 @@ local clear, insert = helpers.clear, helpers.insert
 local command = helpers.command
 local meths = helpers.meths
 local iswin = helpers.iswin
-local nvim_dir = helpers.nvim_dir
+local testprg = helpers.testprg
 local thelpers = require('test.functional.terminal.helpers')
 
 describe('ext_hlstate detailed highlights', function()
@@ -191,7 +191,7 @@ describe('ext_hlstate detailed highlights', function()
       [6] = {{foreground = tonumber('0x40ffff'), fg_indexed=true}, {5, 1}},
       [7] = {{}, {{hi_name = "MsgArea", ui_name = "MsgArea", kind = "ui"}}},
     })
-    command('enew | call termopen(["'..nvim_dir..'/tty-test"])')
+    command(("enew | call termopen(['%s'])"):format(testprg('tty-test')))
     screen:expect([[
       ^tty ready                               |
       {1: }                                       |

test/functional/ui/inccommand_spec.lua

@@ -17,7 +17,7 @@ local source = helpers.source
 local poke_eventloop = helpers.poke_eventloop
 local nvim = helpers.nvim
 local sleep = helpers.sleep
-local nvim_dir = helpers.nvim_dir
+local testprg = helpers.testprg
 local assert_alive = helpers.assert_alive
 
 local default_text = [[
@@ -2875,7 +2875,7 @@ it(':substitute with inccommand during :terminal activity', function()
     clear()
 
     command("set cmdwinheight=3")
-    feed([[:terminal "]]..nvim_dir..[[/shell-test" REP 5000 xxx<cr>]])
+    feed(([[:terminal "%s" REP 5000 xxx<cr>]]):format(testprg('shell-test')))
     command('file term')
     feed('G')  -- Follow :terminal output.
     command('new')

test/functional/ui/output_spec.lua

@@ -9,6 +9,7 @@ local feed_command = helpers.feed_command
 local iswin = helpers.iswin
 local clear = helpers.clear
 local command = helpers.command
+local testprg = helpers.testprg
 local nvim_dir = helpers.nvim_dir
 local has_powershell = helpers.has_powershell
 local set_shell_powershell = helpers.set_shell_powershell
@@ -54,7 +55,7 @@ describe("shell command :!", function()
     if 'openbsd' == helpers.uname() then
       pending('FIXME #10804')
     end
-    child_session.feed_data(":!"..nvim_dir.."/shell-test REP 30001 foo\n")
+    child_session.feed_data((":!%s REP 30001 foo\n"):format(testprg('shell-test')))
 
     -- If we observe any line starting with a dot, then throttling occurred.
     -- Avoid false failure on slow systems.
@@ -207,12 +208,7 @@ describe("shell command :!", function()
 
     it('handles multibyte sequences split over buffer boundaries', function()
       command('cd '..nvim_dir)
-      local cmd
-      if iswin() then
-        cmd = '!shell-test UTF-8  '
-      else
-        cmd = '!./shell-test UTF-8'
-      end
+      local cmd = iswin() and '!shell-test UTF-8  ' or '!./shell-test UTF-8'
       feed_command(cmd)
       -- Note: only the first example of split composed char works
       screen:expect([[

test/functional/ui/searchhl_spec.lua

@@ -5,7 +5,7 @@ local command = helpers.command
 local feed_command = helpers.feed_command
 local eq = helpers.eq
 local eval = helpers.eval
-local nvim_dir = helpers.nvim_dir
+local testprg = helpers.testprg
 
 describe('search highlighting', function()
   local screen
@@ -305,7 +305,7 @@ describe('search highlighting', function()
   end)
 
   it('is preserved during :terminal activity', function()
-    feed([[:terminal "]]..nvim_dir..[[/shell-test" REP 5000 foo<cr>]])
+    feed((':terminal "%s" REP 5000 foo<cr>'):format(testprg('shell-test')))
 
     feed(':file term<CR>')
     feed('G')  -- Follow :terminal output.

test/functional/ui/wildmode_spec.lua

@@ -7,7 +7,7 @@ local meths = helpers.meths
 local eq = helpers.eq
 local eval = helpers.eval
 local retry = helpers.retry
-local nvim_dir = helpers.nvim_dir
+local testprg = helpers.testprg
 
 describe("'wildmenu'", function()
   local screen
@@ -114,7 +114,7 @@ describe("'wildmenu'", function()
   it('is preserved during :terminal activity', function()
     command('set wildmenu wildmode=full')
     command('set scrollback=4')
-    feed([[:terminal "]]..nvim_dir..[[/shell-test" REP 5000 !terminal_output!<cr>]])
+    feed((':terminal "%s" REP 5000 !terminal_output!<cr>'):format(testprg('shell-test')))
     feed('G')  -- Follow :terminal output.
     feed([[:sign <Tab>]])   -- Invoke wildmenu.
     -- NB: in earlier versions terminal output was redrawn during cmdline mode.