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These are all separate but different unittests that have grown
organically apart from each other, combine into a single typed test
so that future file stream reader unittests can reuse this structure.

The one unfortunate part about doing it this way is that it requires a
lot more `this->` everywhere because of C++ templates.  However,
the alternative of having a test fixture with a bunch of switch
statements for the different types seems strictly worse.

Change-Id: Id56bc28391e1a640748691cb383a19291c47fc51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406718
Commit-Queue: Nick Harper <nharper@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807092}