test: Fix tests running in Node 6.0

[keithamus]

Symbol.toStringTag is an available read only property in Node 6. The tests overwrite the property
and as such were failing in Node 6.0. This fixes the tests so that they don't assign on Symbol
if the property already exists.

It also alters the tests to use Symbol.toStringTag proper, rather than the sham string - so that
when toStringTag actually exists, it is actually used rather than the sham; allowing tests to pass
in environments which support toStringTag for real.

@lucasfcosta @davelosert if you could please check this over and merge when happy 😄

[coveralls]

Coverage remained the same at 100.0% when pulling c1bb90b on keithamus:fix-node-6 into 42cd787 on chaijs:master.

[lucasfcosta]

The whole logic looks correct to me, it really fixes what you've described.
However, I've tried to find more about __@@toStringTag__ and I couldn't find info on it, would you mind giving me any links to check on it? Just because I feel like I should learn about how this works exactly.

[keithamus]

__@@toStringTag__ was just a hacky way to sham the behaviour of Symbol.toStringTag, it has no bearing in the real world.

[lucasfcosta]

@keithamus ah, makes total sense, I should've read the whole code, sorry for that
Anyway, good job, Keith!
@davelosert can merge as soon as he wants to 😄

[davelosert]

@lucasfcosta @keithamus : Sorry for the delayed response - there is lots on my desk right now.
This looks very good for me - thanks for the fix @keithamus.

The lgtm is still pending though - should I merge this manually anyways?

LGTM

[keithamus]

I think because @lucasfcosta didn't actually say LGTM it is still pending 😉

[keithamus]

And now its not, because I said LGTM 😆

[davelosert]

@keithamus : Perfect and thanks! 😄

[lucasfcosta]

Oh sorry, I totally forgot about LGTM
Great work guys 👍