patch 9.0.0111: "nocombine" is missing from synIDattr()

Problem:    "nocombine" is missing from synIDattr().
Solution:   Add "nocombine". (Muni Tanjim, closes #10816)
diff --git a/src/testdir/test_syn_attr.vim b/src/testdir/test_syn_attr.vim
index 366f39f..bf4dfbc 100644
--- a/src/testdir/test_syn_attr.vim
+++ b/src/testdir/test_syn_attr.vim
@@ -22,8 +22,10 @@
   call assert_equal('1', synIDattr(hlID("Mine"), "underdotted", 'cterm'))
   call assert_equal('1', synIDattr("Mine"->hlID(), "underdashed", 'gui'))
 
-  hi Mine gui=strikethrough
+  hi Mine term=nocombine gui=strikethrough
   call assert_equal('1', synIDattr(hlID("Mine"), "strikethrough", 'gui'))
+  call assert_equal('1', synIDattr(hlID("Mine"), "nocombine", 'term'))
+  call assert_equal('', synIDattr(hlID("Mine"), "nocombine", 'gui'))
   hi Mine term=NONE cterm=NONE gui=NONE
   call assert_equal('', synIDattr(hlID("Mine"), "bold", 'term'))
   call assert_equal('', synIDattr(hlID("Mine"), "italic", 'cterm'))