[SMTChecker] Tune Spacer options

Some Spacer options work well in different scenarios, and it's hard to tell in advance which options to use. We could have a tuning mechanism that enables/disables options in at least two different runs, if the first run times out/returns unknown.

The issue names SMTChecker and Spacer but no files, tests, or entry points. Start by locating the SMTChecker integration with Spacer and how timeout or unknown results are handled; done means defining and implementing a tuning mechanism that tries different option sets across runs.

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