TEST: Add TODO segments in test/recipes/15-test_genec.t

TEST: Add TODO segments in test/recipes/15-test_genec.t

There currently do not support 'ec_param_enc:explicit' with provider
side key generation.  Reflect that by encoding the expected failure
with a Test::More TODO section for those particular tests.

Because the tests in this recipe are data driven, we implement this
mechanism with two functions, one for stuff that's supported and one
for stuff that isn't.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12080)