Update chai to 4.x

[danez]

Q	A
Patch: Bug Fix?	n
Major: Breaking Change?	n
Minor: New Feature?	n
Deprecations?	n
Spec Compliancy?	n
Tests Added/Pass?	n
Fixed Tickets
License	MIT
Doc PR
Dependency Changes

I had to change how chai gets required.

// in test file
assert.throws(fn, /test/)

chai would check if the second param is instanceOf RegExp, but this would be false as chai was required outside the testContext, so it is referencing a different global RegExp.
By requiring chai inside the testContext fixed this problem.