[Rust] fix Miri flag passing

[RalfJung]

Miri flag passing changed a while ago and the compatibility code is scheduled to be removed soon. This ports the test suite to the new way of passing flags to Miri.

Cc @CasperN

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[RalfJung]

I'm sorry but signing a CLA is too much effort for such a one-line drive-by fix.^^ And moreover it looks like that requires a Google account, which I am not willing to create as it asks for personal data [phone numbers] that I do not want to share.
@CasperN feel free to re-submit this PR as your own if that helps.

[CasperN]

yea I'll resubmit this PR, thanks