Add warning options to the cppsuite files

Context:
Through looking at the coverity issues around the CPP test suites WT-8168, I discovered that we don't have flags that allow us to generate compiler warnings. I found that we don't append CFLAGS when compilng cxx files. This is can reflected in cmake build system through

    if(NOT CREATE_TEST_CXX)

        # Don't append the strict diagnostic flags to C++ targets (as these are chosen for C targets).

        set(test_c_flags "${COMPILER_DIAGNOSTIC_FLAGS}")

    endif()

Inside the autoconf build system this is a little harder to parse:

ac_ext=cpp

ac_cpp='$CXXCPP $CPPFLAGS'

ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'

ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'

ac_compiler_gnu=$ac_cv_cxx_compiler_gnu

I don't entirely understand autoconf build system myself, so I will try my best to explain what is happening. This code represents how it compiles for a cpp file. The --enable-strict generates the warning flag for only CFLAGS. In this case, it seems that we don't add in {$CFLAGS}, which means we skip generating the warning flags.

Task:
This ticket aims to find a solution where we can add warning flags when compiling CPP tests locally. This will require some investigating if we can utilise the CFLAGS that we generate from --enable-strict or if we need to create a seperate flag specifically for CPP test suites. Once compiler warnings are enabled, there was a lot of warnings that were generated that needs to be addressed.

We also want evergreen to enable this warning flag to catch the warnings during pull request builds.