No tests for verifying basic functioning of RecyclerPool implementations (#1064)

[cowtowncoder]

(note: spun off of FasterXML/jackson-databind#4321)

One major new feature in Jackson 2.16 was addition of pluggable RecyclerPools (see #1064 f.ex).
But unfortunately their functioning was not tested beyond regression testing for the default, ThreadLocal-based legacy RecyclerPool.
Let's start by adding streaming-level testing first.

EDIT: there is actually one test -- BufferRecyclerPoolTest -- that verifies some aspects but is limited (only simple generation case, not parser), but we can do better.