Fix various byte compilation warnings in rust-mode-tests.el.

[phst]

• Migrate to lexical binding.
• Declare functions that are conditionally-defined.
• Migrate off the deprecated ‘cl’ library.
• Don’t use ‘setq’ to introduce global variables and constants.
• Avoid “unused lexical variable” warnings.
• Remove some unused variables.
• Stop using interactive-only commands.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @mookid (or someone else) soon.

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[mookid]

Thanks.