Fix for duplicate entries in mocha.options.globals

[pascalpp]

Fixes #3933

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

Adds a deduping reducer to Mocha.prototype.globals to ensure that no duplicate items are added.

Alternate Designs

Decided against using Array.prototype.includes, opted to use IndexOf instead, to ensure the solution has a broad base of support since mocha runs in a lot of places. The method is already using Array.prototype.filter, so I think it's safe to use Array.prototype.reduce, which I believe has equivalent support.

Why should this be in core?

Slight change to a core method of the Mocha constructor API. Not a candidate for its own package.

Benefits

Mostly just house-cleaning.

Possible Drawbacks

Possible speed impact, but probably negligible.

Applicable issues

#3933
Originated from a discussion with @juergba here: #3914

[pascalpp]

@juergba Here's that PR I promised you to fix the duplicate globals issue. Let me know if you think this is the right approach.

[coveralls]

Coverage increased (+0.08%) to 92.429% when pulling 74a9896 on pascalpp:issue/3933 into 29b7615 on mochajs:master.

[pascalpp]

Will attempt to resolve this in #3914