build: make CMake test flags more consistent with make

[jmayclin]

Description of changes:

1. synchronize CMake test build flags with make build flags
   The default make flags contain things like -Wunused which will error on unused variables (good!) but since I don't develop with make I don't see these errors until they run in CI (not good!)

2. add an option to error on warnings in tests
   The CMake build does not currently expose a way to error on compiler warnings in tests. S2N_WERROR_ALL sets the -Werror setting to public on libs2n, so anything that links against libs2n will automatically get the werror setting applied by CMake.

3. fix existing warnings.
   There were enum versions and cast alignment warnings. I fixed those.

/home/ec2-user/workspace/s2n-tls/tests/unit/s2n_signature_algorithms_test.c:242:68: warning: implicit conversion from enumeration type 's2n_authentication_method' to different enumeration type 's2n_pkey_type' [-Wenum-conversion]
                    conn->handshake_params.client_cert_pkey_type = S2N_AUTHENTICATION_ECDSA;
                                                                 ~ ^~~~~~~~~~~~~~~~~~~~~~~~
/home/ec2-user/workspace/s2n-tls/tests/unit/s2n_signature_algorithms_test.c:259:68: warning: implicit conversion from enumeration type 's2n_authentication_method' to different enumeration type 's2n_pkey_type' [-Wenum-conversion]
                    conn->handshake_params.client_cert_pkey_type = S2N_AUTHENTICATION_RSA;
                                                                 ~ ^~~~~~~~~~~~~~~~~~~~~~

I also had to change the s2n_build_test to remove the use of strcasestr, which was causing an implicit-declaration warning on certain platforms like macOS. This is because I removed the -Wno-implicit-function-declaration setting on our unit tests, which will prevent odd kinds of errors like this.

/Users/runner/work/s2n-tls/s2n-tls/tests/unit/s2n_build_test.c:87:29: error: implicit declaration of function 'strcasestr' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            EXPECT_NOT_NULL(strcasestr(ssleay_version_text, name));
                            ^
/Users/runner/work/s2n-tls/s2n-tls/tests/unit/s2n_build_test.c:87:29: note: did you mean 'strcasecmp'?

Testing:

removed an EXPECT_OK in a test, and then confirmed that the following build failed

alias strict='
rm -rf build
cmake -B build . -D CMAKE_C_COMPILER=clang -D CMAKE_BUILD_TYPE=RelWithDebInfo -D S2N_WERROR_ALL=ON &&
cmake --build ./build -j $(nproc) &&
CTEST_PARALLEL_LEVEL=$(nproc) make -C build test ARGS="--output-on-failure"
'

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[goatgoose]

Should this be set in s2n_codebuild.sh for unit tests?

[jmayclin]

Yes, but that is a much larger project unfortunately. That was my initial approach, but I encountered some of the following problems

• 32 bit build has a rollover warning in the 2050 detection function (which will never be called, but the time_t definition has that problem regardless
• individual librypto builds have problems with unused variable/unused function warnings, because there are some functions that never get called under certain if/def conditions.