Introduce the `none.carbon` min-prelude (#5694)
The none.carbon min-prelude is not just an empty prelude, it also
prevents any prelude from being imported at all. So no import machinery
runs before the test, only the `package` statement from the prelude
would run.
Use the none.carbon min-prelude in a few tests that were specifying
`--no-prelude-import` to give it a trial run.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>