Rewrite fileinput tests to use std::rt::io

These tests are being very flaky on the bots, and the reason is that files are
being created and then when attempted to get read they actually don't exist. I'm
not entirely sure why this is happening, but I also don't fully trust the
std::io implemention using @-boxes to close/flush/write files at the right time.

This moves the tests to using std::rt::io which is hopefully more robust and
something that we can actually reason about. Sadly, due to #8810, these tests
fail on windows, so they're all ignored on windows right now.

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