The new structure is heavily inspired by recent work to refactor the way
`lint` and `fmt` batch inputs. I don't expect any behavior change from
this diff - all plugins providing a `test` implementation are coded to
return single-element partitions. Once the plumbing has merged to `main`
I plan to follow-up with an implementation of `pytest` batching.

Known TODOs:
  - Restore the warning/error raised if no applicable targets are found
    for the input specs
  - Restore use of environments in this flow

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I deleted a handful of tests when I was getting started because I
thought they would no longer be relevant. Turns out they continue to
work with a few tweaks, so we can restore them to avoid losing coverage.

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Tracking the name of the test tool didn't really give us much - delete
it, and simplify the code that used to use it.

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Hard-code a value of 1 for now where it was being used.

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Look up the environment name for each element in a test partition, and
assert that everything in a partition matches.

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Add a separate field for `extra_output_prefix` for clarity

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This is currently only used by `pytest`, and we have a couple other
fields with the same problem (de-duplicating between test runs) that we
currently expect plugin implementers to handle. Incidentally, deleting
the field flushed out some invalid `SubPartition`s built in tests that
snuck through the previous cleanup commit.

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