fix compiler warning on signed/unsigned comparison

The warning is given by GCC 4.7 with Boost 1.53.0 and is reported as a
build error, see http://my.cdash.org/viewBuildError.php?type=0&buildid=585601

In file included from /local/modules/boost/include/boost/test/unit_test.hpp:19:0,
                 from /.../src/test/integration/validation/thermodynamics.cpp:23:
/local/modules/boost/include/boost/test/test_tools.hpp: In Instanziierung von »boost::test_tools::predicate_result boost::test_tools::tt_detail::equal_impl(const Left&, const Right&) [mit Left = int; Right = unsigned int]«:
/local/modules/boost/include/boost/test/test_tools.hpp:560:40:   erfordert durch »boost::test_tools::predicate_result boost::test_tools::tt_detail::equal_impl_frwd::call_impl(const Left&, const Right&, mpl_::false_) const [mit Left = int; Right = unsigned int; mpl_::false_ = mpl_::bool_<false>]«
/local/modules/boost/include/boost/test/test_tools.hpp:575:56:   erfordert durch »boost::test_tools::predicate_result boost::test_tools::tt_detail::equal_impl_frwd::operator()(const Left&, const Right&) const [mit Left = int; Right = unsigned int]«
/local/modules/boost/include/boost/test/test_tools.hpp:523:1:   erfordert durch »bool boost::test_tools::tt_detail::check_frwd(Pred, const boost::unit_test::lazy_ostream&, boost::test_tools::const_string, std::size_t, boost::test_tools::tt_detail::tool_level, boost::test_tools::tt_detail::check_type, const Arg0&, const char*, const Arg1&, const char*) [mit Pred = boost::test_tools::tt_detail::equal_impl_frwd; Arg0 = int; Arg1 = unsigned int; boost::test_tools::const_string = boost::unit_test::basic_cstring<const char>; std::size_t = long unsigned int]«
/.../src/test/integration/validation/thermodynamics.cpp:69:1:   von hier erfordert
/local/modules/boost/include/boost/test/test_tools.hpp:536:20: