Use gen-certs.sh for cmake as well

[opoplawski]

Fixes #1798 for cmake. This also runs the tests in the test/ directory for cmake.

[yhirose]

@opoplawski thanks for the pull request.
@sum01 @jimmy-park could you review it for me?

[sum01]

I guess technically the tests run with Cmake will no longer cross-platform since it uses the bash script. I assume no one really cares though.

LGTM, all tests passed on my machine.

[jimmy-park]

If HTTPLIB_IS_USING_OPENSSL and HTTPLIB_TEST are ON on some Windows machines (my machine, actually), CMake configuration will fail.

[yhirose]

@opoplawski could you make an adjustment according to @jimmy-park's comment?

[opoplawski]

Hmm, for cross platform support I think the meson style approach will be needed here. Unfortunately I don't have the time or interest for that so I'll leave that to someone else. Hopefully you can use the _OPENSSL_VERSION cmake variable for checks.