test(insertStyle): ensures that tests are run in sequence

[marcalexiei]

Followup of #162 (comment)

coverage went down for insertStyle function - maybe you can add coverage for it in your next PR (it appears the early return statement isn't covered (https://coveralls.io/builds/70199789)).

I made all insertStyle tests run in sequence.
They need to be executed this way to ensure that the global scope is properly cleared after the test in the DOM environment.
I also added 2 assertions to be sure that only one <style /> is present inside the DOM during the test.

This should bring back insertStyle coverage to his original value.

[elycruz]

Makes sense (👍 ).