Tests for constant propagation of append(Object)

[danpoe]

• 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.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov-io]

Codecov Report

Merging #5122 into develop will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5122      +/-   ##
===========================================
+ Coverage    66.95%   66.99%   +0.04%     
===========================================
  Files         1145     1145              
  Lines        93650    93650              
===========================================
+ Hits         62699    62745      +46     
+ Misses       30951    30905      -46

Flag	Coverage Δ
#cproversmt2	42.68% <ø> (ø)	⬆️
#regression	63.49% <ø> (+0.04%)	⬆️
#unit	31.7% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...lvers/strings/string_constraint_generator_main.cpp	85.05% <0%> (+0.57%)	⬆️
...rs/strings/string_constraint_generator_valueof.cpp	100% <0%> (+19.39%)	⬆️

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 906c2f3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/128502450