(#6407) Fix spec test hang with Mocha >= 0.9.11 in zlib testing

We had a combination of bad logic, and bad testing, and a nasty behaviour of
Mocha <= 0.9.10 that would result in a false pass for one of our tests.

This not only falsely passed, but hid an infinite loop retrying decompression
on an invalid data stream; it could be triggered by anything that sent an HTTP
request with an invalid compressed body, resulting in a livelock.

Paired-with: Jesse Wolfe <jesse@puppetlabs.com>
Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>