Reorganize "low level" tests into separate modules

[drdavella]

These tests have been separated into three different modules in order to make things easier to maintain: tests for the underlying file format, tests for array blocks, and tests for higher-level API features.

[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

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[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

If there are any issues with this message, please report them here.

[coveralls]

Coverage increased (+0.007%) to 94.774% when pulling 443acaa on drdavella:reorganize-tests into cfa6a03 on spacetelescope:master.