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Re-working the tests for PostAsyncResults to by 2 typed tests run on 3
types, rather than 6 stand-alone tests. This reduces the code
duplication without reducing the readability of the tests, and sets up
well for adding additional methods to post_async_results that should be
tested over the same set of types.

For examples of the kinds of additional functions anticipated, see
these abandoned crs:
https://crrev.com/c/1981255
https://crrev.com/c/2386099

Adding said additional functions will be pursued as an independent
change.

Change-Id: I92c46760de15d6876eb73f7f3b49135b1d1570c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406492
Commit-Queue: Hoch Hochkeppel <mhochk@microsoft.com>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806311}