test: recent nekohtml versions do not consider 'a' to be inline

so let's pick an unambiguously-inline element to test.

test/html4/test_element_description.rb

@@ -43,7 +43,7 @@ def test_deprecated?
       end
 
       def test_inline?
-        assert_predicate(ElementDescription["a"], :inline?)
+        assert_predicate(ElementDescription["strong"], :inline?)
         refute_predicate(ElementDescription["div"], :inline?)
       end