OmeroCpp: Add portability fixes for gtest

Some combinations of gtest and the C++ standard library result in
compile failure due to the objects being tested not having
stream output operators.  Change these assertions to avoid
stream output.

The specific failures being addressed here are building develop OmeroCpp with the cmake PR (#2797) merged in, with the gtest/toolchain on CentOS6 (gretzky35).  The jenkins jobs don't actually test this due to using either embedded gtest (current latest build) or gtest directly from svn (current merge build).