test: introduce helpers for skips and for a valgrind block

test/helper.rb

@@ -138,6 +138,21 @@ def stress_memory_while(&block)
       end
     end
 
+    def skip_unless_libxml2(msg="this test should only run with libxml2")
+      skip(msg) unless Nokogiri.uses_libxml?
+    end
+
+    def skip_unless_jruby(msg="this test should only run with jruby")
+      skip(msg) unless Nokogiri.jruby?
+    end
+
+    def refute_valgrind_errors
+      skip_unless_libxml2("valgrind tests should only run with libxml2")
+      yield.tap do
+        GC.start(full_mark: true) if GC_LEVEL == "minor"
+      end
+    end
+
     def assert_indent(amount, doc, message = nil)
       nodes = []
       doc.traverse do |node|