Add failing structure_list2 decoder test

[link2xt]

Test passes if added into official https://github.com/hashicorp/hcl
decoder test.

What pyhcl makes is obviously wrong:
{'top': [{'a': 'a', 'b': 'b', 'c': 'c'}, {'b': 'b'}]} != {'top': [{'a': 'a', 'b': 'b'}, {'b': 'b', 'c': 'c'}]}

No solution yet, but looks like the problematic code was added in 46a4138

The code erroneously adds 'c' into the first dictionary, but then discovers 'b' that already exists and extends the list, so the 'c' key appears moved to the first dictionary.

Travis CI will likely fail this one, but please merge it in.

[virtuald]

If someone pushes a PR to fix the failing test, I'm happy to merge it. I'll keep the PR open, but I don't personally have bandwidth to address it. Maybe @scottbelden ?

[link2xt]

Fixed structure_list2 test with a fairly straightforward fix, but added structure_list3