Running Buidler's test suite in our CI

[cameel]

From #9237 (comment):

This is now ready on our side. All you need to do to run our test suite with a custom compiler is:

git clone [email protected]:nomiclabs/hardhat.git
cd hardhat
git checkout development
HARDHAT_TESTS_SOLC_VERSION="0.7.0" HARDHAT_TESTS_SOLC_PATH="/some/path/soljson-v0.7.0+commit.9e61f92b.js" scripts/run-tests-with-custom-solc.sh

(Note the version and path in the last command)

This will install all the necessary dependencies and run the relevant tests. The only requirement is node.js. Any of the currently actively maintained versions would do it.

If the tests fail, the script will exit with a non-zero status number.

EDIT: I slightly modified the quote above to use the updated snippet from #9237 (comment).

We should hook it up to run in our CI. To run either on every PR (if it takes no more than 15 min) or in the nightly run (if it takes more).

The setup should be similar to what we do in test/externalTests/ but there's no need to fork the repo in solidity-external-test - it should check out Buidler's repo directly.

These tests should be running also on the breaking branch.