Fix memory leak in context unit test
Fix memory leak in context unit test

Before fix, `valgrind ./tests 1` outputs
  ==21959==     in use at exit: 2,228,288 bytes in 8 blocks

after:
  ==23974==     in use at exit: 0 bytes in 0 blocks

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