Add coverage reporting to the test suite

[kurtmckee]

This PR introduces the following changes

• Add coverage reporting to the test suite and in CI
• Fail the test suite if coverage falls below 94% (its current value for individual runs)
• Support parallel test suite execution locally (run tox run-parallel, or just tox p)

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Key elements to add to the test suite:

Caching tests

ETag header generation

Vary header generation