Some tests mark clusters as 'dirty', which makes them non-reusable by
later tests; we don't want to return them to the pool of clusters.
This use-case was covered by the `add_one` function in the `Pool` class.
However, it had the unintended side effect of creating extra clusters
even if there were no more tests that were waiting for new clusters.
Rewrite the implementation of `Pool` so it provides 3 interface
functions:
- `get` borrows an object, building it first if necessary
- `put` returns a borrowed object
- `steal` is called by a borrower to free up space in the pool;
the borrower is then responsible for cleaning up the object.
Both `put` and `steal` wake up any outstanding `get` calls. Objects are
built only in `get`, so no objects are built if none are needed.
Closes#11558
348582c4c8