Run test on Windows

[feugy]

Hello!

I'd like to give a try on instrumenting assert.throws() as requested here.
While digging into the power-assert galaxy, I realised I couldn't run the tests on a Windows OS.

This PR makes it possible to run all tests on such environment.

[coveralls]

Coverage remained the same at 97.778% when pulling 76d0247 on feugy:master into c624cbb on power-assert-js:master.

[twada]

@feugy Ah nice catch. Thanks!