patch 8.0.0958: terminal test fails on Windows when winpty dll is missing

Problem:    The terminal test fails on MS-Windows when compiled with the
            terminal feature but the winpty DLL is missing.
Solution:   Check if the terminal feature works. (Ken Takata)

src/testdir/test_terminal.vim

@@ -1,6 +1,6 @@
 " Tests for the terminal window.
 
-if !exists('*term_start')
+if !has('terminal')
   finish
 endif
 

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    958,
 /**/
     957,
 /**/