Env's (like the POSIX Env) which use an actual filesystem behave
differently than InMemoryEnv with regards to writing data to a currently
open file.

InMemoryEnv::NewWritableFile would previously delete that file,
if it was open, before creating a new file so any previously
open file would be unlinked. This change truncates an open file
so that subsequent reads will read that new data.

This should have no impact on leveldb as it never has the same
file open for both read and write access. This change is only
being made for tests (specifically a future change to corruption_test)
to allow them to be decoupled from the underlying platform and
allow them to use an Env.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237858231