drop new constants from test_api and usages from test_core, re-publish test_core (#1688)

It turns out test_core did rely on some changes in test_api that weren't documented in the changelog, I yanked version 0.4.12, and this will re-publish it as 0.4.13.

We don't want to do a release in test_api because flutter users won't pick up that version for a long time (on stable at least), so this removes the reliance on those new apis as well.
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