Motivation:
We observed the testThreadCacheDestroyedByThreadCleaner test failing due
to timing out after 10 seconds, while in the `runFinalization` call.

After analyzing the code, I've determined that the most likely cause is
that the `runFinalization` call itself is very slow, because it waits
for the processing of _all_ eligible finalizers, rather than just the
one we care about.

Modification:
- Remove the call to `runFinalization` and instead rely on the
background reference processing thread.
- Increase the timeout from 10 to 20 seconds, to account for a busier
runtime environment now that we run all the buffer tests in parallel.

Result:
More stable build, hopefully.

(cherry picked from commit 1e78d3f)