Add: flag in nightwatch cli

[vaibhavsingh97]

Fixes #3231

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Output after using --list-files flag:

[gravityvi]

Can we also write an option for this CLI argument to display it in the nightwatch help command and in nightwatch documentation?

[harshit-bs]

@vaibhavsingh97 Please add the description of the --list-files flag here as well

[harshit-bs]

can you add tests as well?