patch 8.0.0603: gF test fails on MS-Windows

Problem:    gF test fails on MS-Windows.
Solution:   Use @ instead of : before the line number
diff --git a/src/testdir/test_gf.vim b/src/testdir/test_gf.vim
index 1f3db24..d5bdfab 100644
--- a/src/testdir/test_gf.vim
+++ b/src/testdir/test_gf.vim
@@ -38,7 +38,7 @@
   w! Xfile
   close
   new
-  call setline(1, ['one', 'Xfile:3', 'three'])
+  call setline(1, ['one', 'Xfile@3', 'three'])
   2
   call assert_fails('normal gF', 'E37:')
   call assert_equal(2, getcurpos()[1])