Tests for constant propagation of StringBuilder.append()

[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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 6ecf35e).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/128152963

[codecov-io]

Codecov Report

Merging #5114 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #5114      +/-   ##
===========================================
+ Coverage    69.71%   69.71%   +<.01%     
===========================================
  Files         1322     1322              
  Lines       109429   109430       +1     
===========================================
+ Hits         76287    76288       +1     
  Misses       33142    33142

Impacted Files	Coverage Δ
...c/java_bytecode/java_string_library_preprocess.cpp	96.81% <100%> (ø)	⬆️

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