Add unit tests for get_user_specified_clinit_body

[romainbrenguier]

Based on #5078

• Each commit message has a non-empty body, explaining why the change was made.
• [na] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [na] 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).
• [na] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• [na] White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 5d91b1f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/126215997

[codecov-io]

Codecov Report

Merging #5081 into develop will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #5081      +/-   ##
===========================================
+ Coverage    69.66%   69.67%   +0.01%     
===========================================
  Files         1319     1319              
  Lines       109250   109290      +40     
===========================================
+ Hits         76106    76150      +44     
+ Misses       33144    33140       -4

Impacted Files	Coverage Δ
unit/testing-utils/expr_query.h	100% <ø> (+20%)	⬆️
...a_static_initializers/java_static_initializers.cpp	100% <100%> (ø)	⬆️
...bmc/src/java_bytecode/java_static_initializers.cpp	99.1% <0%> (+0.89%)	⬆️

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