build: search for first match between libcoap-3 and libcoap-3-openssl
some libcoap installation has libcoap-3.pc pkgconfig file that symlinks to a specific variant, others don't. Therefore searching for the first successful match between libcoap-3 and libcoap-3-openssl.
This allows other libcoap-3 variants to be used when libcoap-3-openssl is not found.
cmake: add flag to skip tests build
Allowing to skip gtests when not necessary