Add array/hash implicit allocation tests

These are designed to prevent allocation regressions (commits that
increase the number of implicitly allocated arrays and hashes). We
have already had three commits in the last couple weeks to fix
allocation regressions:

* 15dc3aaa311b32203d8ffb414bcf9b8e55ce5691
* aceee71c35e0b387691836e756b4e008efd84cf1
* c38878494377c94f2425a81e598260ea944ef7f3

This test suite should hopefully allow us to find such regressions
in CI before commit, to avoid committing future allocation regressions.

This uses assert_separately around each set of tests.  Doing it for
each individual check was too slow.  Failures are gathered and
reported at the end of the the suite as a single assertion, with
the message describing all failures.

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