vim-patch:8.0.1179: Test_popup_and_window_resize() does not always pass

Problem:    Test_popup_and_window_resize() does not always pass.
Solution:   Do not use $VIMPROG, pass the Vim executable in the vimcmd file.
            (Ozaki Kiichi, closes vim/vim#2186)
631820536e

vim-patch:8.0.1526: no test using a screen dump yet

Problem:    No test using a screen dump yet.
Solution:   Add a test for C syntax highlighting.  Add helper functions.
da65058a9c

NOTE: uses modified `GetVimProg()` (which is used with skipped tests only
(mostly because of `!has('terminal')`)).

Vim uses a 'vimcmd' file, while Nvim uses `$NVIM_TEST_ARGX` environment
variables.

Ref: https://github.com/vim/vim/pull/4806

src/nvim/testdir/shared.vim

@@ -196,16 +196,26 @@ func s:feedkeys(timer)
   call feedkeys('x', 'nt')
 endfunc
 
+" Get $VIMPROG to run Vim executable.
+" The Makefile writes it as the first line in the "vimcmd" file.
+" Nvim: uses $NVIM_TEST_ARG0.
+func GetVimProg()
+  if empty($NVIM_TEST_ARG0)
+    " Assume the script was sourced instead of running "make".
+    return '../../../build/bin/nvim'
+  endif
+  return $NVIM_TEST_ARG0
+endfunc
+
 " Get the command to run Vim, with -u NONE and --headless arguments.
 " If there is an argument use it instead of "NONE".
-" Returns an empty string on error.
 func GetVimCommand(...)
   if a:0 == 0
     let name = 'NONE'
   else
     let name = a:1
   endif
-  let cmd = v:progpath
+  let cmd = GetVimProg()
   let cmd = substitute(cmd, '-u \f\+', '-u ' . name, '')
   if cmd !~ '-u '. name
     let cmd = cmd . ' -u ' . name
@@ -215,6 +225,14 @@ func GetVimCommand(...)
   return cmd
 endfunc
 
+" Get the command to run Vim, with --clean.
+func GetVimCommandClean()
+  let cmd = GetVimCommand()
+  let cmd = substitute(cmd, '-u NONE', '--clean', '')
+  let cmd = substitute(cmd, '--headless', '', '')
+  return cmd
+endfunc
+
 " Run Vim, using the "vimcmd" file and "-u NORC".
 " "before" is a list of Vim commands to be executed before loading plugins.
 " "after" is a list of Vim commands to be executed after loading plugins.