Add `--parallel` flag for hardhat tests

[schaable]

Describe the feature

In Hardhat 2, we had a --parallel flag for Mocha tests. This flag wasn't ported to Hardhat 3.
In Hardhat 3, we now have two test runners:

• Node.js test runner: runs tests in parallel by default (as of #7311)
• Mocha: can only be configured to run in parallel through the hardhat config file:

const config: HardhatUserConfig = {
  ...,  
  test: {
    mocha: {
      parallel: true,
    },
  },
};

We need to decide how to handle the --parallel flag in Hardhat 3.

Currently, there's an inconsistency where Mocha requires config-based setup while Node.js has parallel as the default behavior.

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