Reduce boilerplate in compiler JUnit tests (#5158)

Many JUnit tests share a compiler instance between all
test cases in a class to reduce overhead.

This commit refactors the mechanism to reduce the boilerplate.

In the new scheme:

  - Using the `@ClassRule` hook in JUnit, we create a per-class
    map for each test class.
  - Per-class values are registered from the test class itself
    by calling `cached("someKey", () => mkExpensiveThing)`
  - At the end of the test, the entries in this map are `close()`-ed
    (if they implement `Closable`), and are released for
    garbage collection.)

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