Add test passing status info for Z3 and CVC4

[allredj]

Add special tags to track the support of string functions by 3rd party solvers Z3 and CVC4.
Running the suite with 3rd party solvers allows to:

• check the sanity of the suite
• track support of string ops by these solvers, when we later benchmark cbmc against those.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• 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).
• 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.

[peterschrammel]

remove that blank line

[allredj]

done

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 9f47f8d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106225418
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.