This test creates a new temp file and writes some bytes into it.
It then opens a new file for the file descriptor of the temp file and
tries to read some bytes out of it.
Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
