This is a mocha caching and cleanup issue addressed here: mochajs/mocha#2783.

Multiple runs of the test task within a hardhat script are now enabled. The resolution is `mocha.dispose` at the end of a run to clear up state.

The mocha dispose method was added in mocha@7.2.0 but the @types/mocha doesn't reflect it till `9.0.0`. This commit:

* bumps mocha as a dep in hardhat-core to 7.2.0 to be explicit
* bumps mocha as a dev-dep everywhere else to 7.2.0 for consistency in our local dev environment
* bumps @types/mocha to 9 in dev-deps in all packages to allow use of dispose and consistency across packages - no code changes are required to support this. Note that the sample typescript project already provides a default @types/mocha on version 9.0.0

Relates to #1720