patch 8.0.1747: MS-Windows: term_start() does not set job_info() cmd

Problem:    MS-Windows: term_start() does not set job_info() cmd.
Solution:   Share the code from job_start() to set jv_argv.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 158bb98..1b87895 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -786,6 +786,12 @@
   let g:job = term_getjob(buf)
   call term_wait(buf, 50)
 
+  if has('win32')
+    call assert_equal('cmd', job_info(g:job).cmd[0])
+  else
+    call assert_equal(&shell, job_info(g:job).cmd[0])
+  endif
+
   " ascii + composing
   let txt = "a\u0308bc"
   call term_sendkeys(buf, "echo " . txt . "\r")