Properly close the writer in unit test.
Properly close the writer in unit test.

Failing to do so generate very spammy logs reminding us that the writer was destroyed without being properly closed.

PiperOrigin-RevId: 372142737
Change-Id: I55c56093c17c12ac3c9468f7919f739d38402cbb

deepmind/reverb,deepmind/reverb,deepmind/reverb,deepmind/reverb