[infrastructure] get unit test runner working again (broken due to rewire-webpack being incompatible with webpack 2)

[gkovacs]

Specifically, rewire-webpack doesn't support webpack 2 (upstream issue: jhnns/rewire-webpack#22 ) and I did not fully succeed at porting it over myself as all the code and relevant APIs are currently undocumented. Will either need to

a) do without unit testing for now and wait for rewire-webpack to fix compatibility with webpack 2

b) downgrade back to webpack 1

c) find another solution for monkey-patching modules for unit testing.