Fix for one of previous commit fixing tests

The parameter of th eintroduced addOptions2url function has to be const,
otherwise it won't build the way this function is called by tests

Author: lawrinn

test/unit/unit_fixture.cpp

@@ -512,7 +512,7 @@ bool unit_fixture::setVariableValue(const sql::SQLString& name, const sql::SQLSt
   return false;
 }
 
-String unit_fixture::addOptions2url(String& add)
+String unit_fixture::addOptions2url(const String& add)
 {
   String localUrl;
   localUrl.reserve(url.length() + 1 + add.length());

test/unit/unit_fixture.h

@@ -363,7 +363,7 @@ class unit_fixture : public TestSuite
   bool setVariableValue(const sql::SQLString& name, const sql::SQLString& value, bool global= false);
 
   /* Adds given options to the base Url on which the tests are run */
-  String addOptions2url(String& add);
+  String addOptions2url(const String& add);
 
 public: