Leaky global regression

If a global variable is whitelisted, it will throw off the length check in `checkGlobals` (a speed optimization made in 747588f92d1113dd0afccf660c85a5e589681c31). 

This pull requests includes two commits: 
1) a test to show the undetected global leak bug
2) a hand-woven revert of 747588f92d1113dd0afccf660c85a5e589681c31

Perhaps there's another way to fix this while still checking the lengths of `globals` and `ok`, but it wasn't obvious to me, and the overhead of doing the full leak check seemed pretty small. 

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As always, thanks so much for making _the_ best javascript test framework.