tests: overhaul to update to 1.10.x patterns and enforce better consistency

[parkedwards]

too often, I find that I'm re-creating acceptance test suites from scratch, so the lead time to do any sort of TDD becomes too long. also this leads to test cases being a bit different from file to file, and then debugging failed tests becomes an archaeological dig. my hypothesis is that this is because we have multiple ways to write tests / fixtures / helpers, and we've also probably got some repeated code that is causing authors to have to reinvent the wheel repeatedly.

since we have a good chunk of test coverage now, I propose that we're able to do a little refactoring to do the following:

1. Starting 1.10.x, the testing framework marked the currently used Check func that we use as "Legacy" + indicates that we should use the new ConfigStateChecks attribute as well as State Check implementations
2. Explore Plan Checks / State Checks
3. implement standardized Import checks (where necessary) - may depend on standardize import parameters schema #335
4. standardize test case formats - naming
5. standardize fixture instantiation - we have a couple of cool methods using go templating thanks to @mitchnielsen , so there should be a way to make the naming and patterns consistent (eg. when do we create new resources for update assertions?)
6. standardize and share custom test checks - we have a couple of custom check functions mainly used to verify if the remote API has the resource object created in the TF test. These are pretty repetitive and basically re-implement Get() logic. We should look to standardize these common ones into shared helpers, but also find a way to codify a pattern so that test authors know exactly how/when to write a new custom check function
7. Add in a TF version check
8. Explore sweepers