Flip passing FUTURE tests to CORE in regression/cbmc-primitives

[NlightNFotis]

During testing for an unrelated feature I noticed that these tests marked
FUTURE are now passing when run under level CORE and have thus flipped
them to that level.

Additionally, I have cleaned up some of the comments and removed references
to internal bug tracker references that might be confusing to outside contributors.

• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6438 (c505c92) into develop (c65874f) will not change coverage.
The diff coverage is n/a.

❗ Current head c505c92 differs from pull request most recent head a987c67. Consider uploading reports for the commit a987c67 to get more accurate results

@@           Coverage Diff            @@
##           develop    #6438   +/-   ##
========================================
  Coverage    76.00%   76.00%           
========================================
  Files         1527     1527           
  Lines       164463   164463           
========================================
  Hits        125008   125008           
  Misses       39455    39455           

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