Use async/await for callbacks in test files

[ccowell]

This issue addresses the need to convert callbacks/Promises into async/await pattern as a refactoring step. The PRs that reference this issue may also include other changes such as updated library paths, variable names/declarations or swapping out arrow functions, etc. The goal is to make the review process as easy as possible.

Note: Will revisit refactoring send functionality requiring a callback. #351

Files reviewed or ready for review

• accounts.js MERGED
• bad_input.js
• blockTags.js
• call.js MERGED
• customGasLimit.js MERGED
• customGasPrice.js MERGED
• debug.js MERGED
• enable_constantinople_hardfork.js MERGED
• ethereum.js MERGED
• ethers.js MERGED
• events.js
• forking.js
• forkingasprovider.js
• forking_deploy_after_fork.js
• gas.js MERGED
• gasLimit.js MERGED
• gasPrice.js MERGED
• hex.js
• interval_mining.js
• keepAliveTimeout.js MERGED
• mining.js
• persistence.js
• public-exports.js MERGED
• requests.js
• runtime_errors.js
• server.js MERGED
• snapshotting.js
• stability.js
• swarm.js MERGED
• time_adjust.js
• transaction.js
• transaction_ordering.js
• transaction_rejection.js
• undefined.js
• unlimited_contract_size.js
• vm.js
• whisper.js MERGED