bin/mocha: don't spawn child process unnecessarily

[boneskull]

This may or may not be a good idea:

If the user has not passed Node.js-specific or V8-specific arguments, import main from lib/cli/cli.js and execute it.

If the user has passed Node.js-specific or V8-specific arguments, use current behavior.

Advantages:

• faster startup time

Disadvantages:

• increased potential for inconsistent behavior
• would require us to be much more careful about which executable we're using in integration tests
• would possibly require the same integration tests execute against both executables

This isn't related to process replacement, but that's something else to consider if the hacks aren't too bad.

[stale]

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[juergba]

closed, see #3827