test: small improvements to memory leak test suite

Related to #1603

test/test_memory_leak.rb

@@ -92,8 +92,11 @@ def test_for_memory_leak
     def test_node_set_namespace_mem_leak
       xml = Nokogiri::XML("<foo></foo>")
       ctx = Nokogiri::XML::XPathContext.new(xml)
-      loop do
-        ctx.evaluate("//namespace::*")
+      20.times do
+        10_000.times do
+          ctx.evaluate("//namespace::*")
+        end
+        puts MemInfo.rss
       end
     end
 
@@ -161,7 +164,7 @@ def test_leak_on_xpath_string_function
       end
     end
 
-    def test_leaking_namespace_node_strings
+    def test_leaking_namespace_node_strings_no_prefix
       # see https://github.com/sparklemotion/nokogiri/issues/1810 for memory leak report
       ns = { "xmlns" => "http://schemas.xmlsoap.org/soap/envelope/" }
       20.times do