test: fix solidity test coverage

[mariocao]

This PR fixes the solidity coverage tests.

Problem:

• Apparently, as we significantly increased the number of tests of this library, the logs to be processed by solidity-coverage were too big.
• The error was: javascript heap out of memory.

Workaround:

• For code coverage check it is not required to test all the curves (we were testing 6 curves).
• We included a environment variable to check if the tests were for code coverage.
• If so, only the secp256k1 curve is tested.

Notes:

• The solidity-coverage documentation provides some complex project examples that use their library, e.g. (OpenZeppelin Contracts)[https://github.com/OpenZeppelin/openzeppelin-contracts].
• Those projects uses solidity-coverage as follows:
  • A forked ganache-cli (ganache-cli-coverage)[https://github.com/frangio/ganache-cli/releases/download/v6.4.1-coverage/ganache-cli-coverage-6.4.1.tgz].
  • A test.sh bash scripts to handle the ganache lifecycle, callinng the aforementioned ganache-cli-coverage with the following arguments: --emitFreeLogs true --allowUnlimitedContractSize true --gasLimit 0xfffffffffffff --port <port>" > /dev/null &.