Update language version to C# 6 for tests

[meaghanlewis]

@Neme12 updated every project under src to use C# 6 here: #1746.

We should go through and do the same for every project under test as well.

[Neme12]

I think when I looked in the test folder I saw some projects that weren't in the solution (or it's possible I didn't find them because they had a different name or directory structure) so I wasn't sure about what to do. Sorry for skipping that!