Revert "Add const for loop variable in regression_test.cc"

This reverts commit af09a15.

The RegressionTest::update_content_lengths method modifies the state
so it cannot be const. Therefore the test here cannot be const.

Author: hnakamur

test/regression/regression_test.cc

@@ -280,7 +280,7 @@ std::unique_ptr<RegressionTests> RegressionTests::from_yajl_node(const yajl_val
 }
 
 void RegressionTests::update_content_lengths() {
-    for (const auto &test : tests) {
+    for (auto & test : tests) {
         test->update_content_lengths();
     }
 }