Add a secp256k1_fe_cmov unit test.
Add a secp256k1_fe_cmov unit test.

(Also add my name to the top of tests.c: I wrote a bunch of that
 and update the copyright dates)

instagibbs/secp256k1,afk11/secp256k1,evoskuil/secp256k1,libbitcoin/secp256k1,apoelstra/secp256k1,kleetus/secp256k1,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,apoelstra/secp256k1,marcusdiaz/BitcoinUnlimited,instagibbs/secp256k1,rustyrussell/secp256k1,sipa/secp256k1,Bitcoin-com/BUcash,martindale/secp256k1,shea256/secp256k1-zkp,chfast/secp256k1,GSongHashrate/secp256k1,voisine/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,digitalbitbox/secp256k1,chfast/secp256k1,Gustav-Simonsson/secp256k1,martindale/secp256k1-zkp,rustyrussell/secp256k1,libbitcoin/secp256k1,peterdettman/secp256k1,digitalbitbox/secp256k1,Justaphf/BitcoinUnlimited,martindale/secp256k1,martindale/secp256k1-zkp,sipa/secp256k1-zkp,pmienk/secp256k1,BitcoinUnlimited/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,voisine/secp256k1,sipa/secp256k1,libmetrocoin/secp256k1,bitcoin/secp256k1,voisine/secp256k1,sipa/secp256k1,sipa/secp256k1-zkp,Bitcoin-com/BUcash,voisine/secp256k1,Gustav-Simonsson/secp256k1,vlajos/secp256k1,apoelstra/secp256k1,01BTC10/secp256k1,rustyrussell/secp256k1,krzysztofwos/BitcoinUnlimited,Bitcoin-com/BUcash,kleetus/secp256k1,01BTC10/secp256k1,libbitcoin/secp256k1,martindale/secp256k1,libbitcoin/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,instagibbs/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,kleetus/secp256k1,Bitcoin-com/BUcash,marcusdiaz/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,BTCfork/hardfork_prototype_1_mvf-bu,chfast/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,vlajos/secp256k1,pmienk/secp256k1,instagibbs/secp256k1,sipa/secp256k1-zkp,vlajos/secp256k1,pmienk/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,afk11/secp256k1,cddjr/BitcoinUnlimited,bitcoin-core/secp256k1,randy-waterhouse/secp256k1,cddjr/BitcoinUnlimited,kleetus/secp256k1,evoskuil/secp256k1,krzysztofwos/BitcoinUnlimited,afk11/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,BitcoinUnlimited/BitcoinUnlimited,bitcoin/secp256k1,BitcoinUnlimited/BitcoinUnlimited,shea256/secp256k1-zkp,apoelstra/secp256k1,ChrisXin/secp256k1,afk11/secp256k1,ChrisXin/secp256k1,martindale/secp256k1,bitcoin/secp256k1,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,libmetrocoin/secp256k1,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,evoskuil/secp256k1,apoelstra/secp256k1,rustyrussell/secp256k1,cddjr/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,Gustav-Simonsson/secp256k1,Bitcoin-com/BUcash,randy-waterhouse/secp256k1,ChrisXin/secp256k1,sipa/secp256k1-zkp,ElementsProject/secp256k1-zkp,sipa/secp256k1-zkp,shea256/secp256k1-zkp,shea256/secp256k1-zkp,Justaphf/BitcoinUnlimited,marcusdiaz/BitcoinUnlimited,krzysztofwos/BitcoinUnlimited,GSongHashrate/secp256k1,kleetus/secp256k1,sipa/secp256k1,bitcoin/secp256k1,digitalbitbox/secp256k1,randy-waterhouse/secp256k1,Bitcoin-com/BUcash,digitalbitbox/secp256k1,cddjr/BitcoinUnlimited,libmetrocoin/secp256k1,chfast/secp256k1,voisine/secp256k1,martindale/secp256k1-zkp,ChrisXin/secp256k1,GSongHashrate/secp256k1,BTCfork/hardfork_prototype_1_mvf-bu,chfast/secp256k1,krzysztofwos/BitcoinUnlimited,libmetrocoin/secp256k1,randy-waterhouse/secp256k1,pmienk/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,Gustav-Simonsson/secp256k1,BitcoinUnlimited/BitcoinUnlimited,evoskuil/secp256k1,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,afk11/secp256k1,GSongHashrate/secp256k1,Justaphf/BitcoinUnlimited,peterdettman/secp256k1,instagibbs/secp256k1,Justaphf/BitcoinUnlimited,ftrader-bitcoinunlimited/hardfork_prototype_1_mvf-bu,krzysztofwos/BitcoinUnlimited,martindale/secp256k1-zkp,ElementsProject/secp256k1-zkp,digitalbitbox/secp256k1,vlajos/secp256k1,bitcoin-core/secp256k1,01BTC10/secp256k1,evoskuil/secp256k1,marcusdiaz/BitcoinUnlimited,cddjr/BitcoinUnlimited,01BTC10/secp256k1,libbitcoin/secp256k1