iotune: close test file on shutdown
iotune: close test file on shutdown

Latest patches added an assertion that all files are properly closed
in their destructors. In order to prevent iotune from failing that
assertion, the test file is properly closed on shutdown.

Refs https://github.com/scylladb/scylla/issues/4946

Message-Id: <e854e7f6a0f9da2df8931c9c9f421bf4000e117b@scylladb.com>

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