patch 9.0.1372: test for 'toolbariconsize' may fail

Problem:    Test for 'toolbariconsize' may fail.
Solution:   Only test 'toolbariconsize' when it is supported. (James McCoy,
            closes #12095)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 7bd4216..b3b90d1 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -1642,6 +1642,8 @@
   endif
   if exists('+toolbar')
     call add(optlist, ['toolbar', 'text', 'a123'])
+  endif
+  if exists('+toolbariconsize')
     call add(optlist, ['toolbariconsize', 'medium', 'a123'])
   endif
   if exists('+ttymouse') && !has('gui')