Add support for testing supported curves

since early versions of 1.0.2 openssl supports -curves command line
option, it allows us to set the curves advertised as supported

use the same approach to testing: advertise all, check what server
accepts, remove the accepted from list, repeat. When server aborts
connection or selects non ECC cipher, we know that we've tested all.
would it be an idea to make this/these types of parameters true booleans instead of strings ?
TEST_CURVES=false