Re-enable or remove "decode too long" and "decode too short" streaming exception tests

[mgsloan]

Due to commercialhaskell/stack#2604 (comment), I took a look at the decodeTooLong and decodeTooShort tests. Looks to me like the issue is that the test assumes Int is 8 bytes, but on ARM it's likely only 4.

I also noticed that these two tests are out of date - they do not include poking the messageMagic. The test is expecting any PeekException, including "Wrong message magic, ...".

I've pushed a branch, https://github.com/fpco/store/tree/alternative-streaming-test-encode, with an alternative approach for generating the too-short message and too-long message. Surprisingly, neither causes an exception to be thrown! Is this a bug?

For now, disabling the tests as all they're testing is the magic number detection.