Add missing test description file

[tautschnig]

When adding the test in 6b09359 I had likely failed to do git add
on test.desc, and instead just added the C file. This commit now
rectifies that.

• 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]

Codecov Report

Merging #6652 (9327fd8) into develop (4c2c27d) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6652   +/-   ##
========================================
  Coverage    76.73%   76.73%           
========================================
  Files         1579     1579           
  Lines       182008   182008           
========================================
+ Hits        139671   139672    +1     
+ Misses       42337    42336    -1     

Impacted Files	Coverage Δ
src/pointer-analysis/value_set.cpp	83.29% <0.00%> (+0.12%)	⬆️

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