Adds test to demo key Symbol in Map.set()

[abacaphiliac]

Adds test to demonstrate usage of Symbol primitive data type as Map key via set method, which does not work upon Map instantiation.

Split from #1392

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[abacaphiliac]

@leebyron please review.

I'm working on another request right now for the Map instantiation performance test(s).

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[leebyron]

Great! thanks :)