Add Mocha benchmark

[mathiasbynens]

Specifically, Mocha seems to perform lots of operations like Array#includes and Array#indexOf on frozen arrays while setting up its environment and executing its tests.

It would be good to have this pattern reflected in the WTB.

Ref. https://docs.google.com/document/d/1X6zO5F_Zojizn2dmo_ftaOWsY8NltPHUhudBbUzMxnc/edit.

[robpalme]

@mathiasbynens Are you looking to solve this by adding a specific Mocha benchmark (as the title says) or would you accept a more focused benchmark (derived from Mocha's actions) that specifically targeted operations on frozen arrays?

[mathiasbynens]

Ideally, we'd add a benchmark that exercises the relevant parts of Mocha.

We're specifically not looking for microbenchmarks (since those don't reflect real-world performance).

[robpalme]

@mathiasbynens We have two volunteers (Damola and Yilang) that want to pair on this. Will be in touch for next steps.