Support Solidity tests being in the test folder

[kanej]

User feedback in the Private Alpha suggests that users expect to be able to put sol files that are tests under the ./test folder. In Hardhat 2 the ./test folder is used for integration tests.

We should support adding Solidity Test sol file under the ./test folder as well as JS/TS integration tests.

This may be done by include ./tests in the source folder lists by default in our configuration. This needs to be thought through.

TODO

• Analysis of what to allow to be configured on testing folders/source folders
• Update of configuration
• Integration test to cover the setup