smt2 bit vector fix [blocks: any PR adding CBMC tests]

[kroening]

This is #3119 plus a commit that enables the SMT2 tests that subsequently pass.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[martin-cs]

Fine modulo the one comment.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 33cfe25).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90470839

[peterschrammel]

I don't quite like the delete script; would prefer adding a tag to the .desc file as done for excluding tests that don't yet run with symex-driven lazy loading, but ok; hope this becomes obsolete soon.

[kroening]

@peterschrammel Let's say we'll get the number of non-working tests below 50, and then we switch to the flag!