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)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index c75d07f..64104ab 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1,6 +1,6 @@
 " Tests for the terminal window.
 
-if !exists('*term_start')
+if !has('terminal')
   finish
 endif