Smt2 strings tests fixes

[allredj]

A couple of files were misnamed and one desc file was missing the -- which caused it to fail.

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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 17b55d1).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106314594
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.