[Nearby] Implement unit tests for InputFile.

Two tests related to Read() have been excluded from running
on Windows due to undiagnosed crashes. We can resolve them
in the future when Nearby launches for Windows devices.

Bug: 1124992
Change-Id: I682cf64604344314c40e276042c08dbb8023c800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402442


Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#805889}