Factor out property decider utilities

[peterschrammel]

so that they can be reused across goto checker implementations and in unit tests.

Does not change behaviour.

• 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.
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[tautschnig]

clang-format wants the above on a single line.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: acc1496).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103884805