v6 CLI arg verification breaks extra args parsing

Hello, currently karma CLI doesn't allow explicit test files as arg so we're passing them at the end and parse them out in `karma.conf.js` and v6 breaks it so I'm essentially asking if:
1. Karma CLI can support explicit test files passed in as args OR
2. Make strict mode be optional

I understand that this is not previously a feature but a lot of stack overflow answers seem to point to this mechanism being the workaround to further modify karma conf using CLI arg.

The way we're calling it is essentially `karma start karma.conf.js [testFiles...]`

Thanks!

Start by tracing v6 CLI argument verification for the `karma start karma.conf.js [testFiles...]` invocation and how trailing arguments are handled before they reach `karma.conf.js`. Determine whether explicit test files or optional strict mode is the intended behavior; done means the chosen behavior works without breaking CLI argument handling.

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