Split unit test into two to avoid GCC warning

[tautschnig]

GCC (version 7.5) issues the following warning: "variable tracking size limit
exceeded with -fvar-tracking-assignments, retrying without" Although harmless,
there is no need to push GCC this far.

• 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 #5802 (a6cf15b) into develop (0dfbd1e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5802   +/-   ##
========================================
  Coverage    69.72%   69.72%           
========================================
  Files         1242     1242           
  Lines       100897   100897           
========================================
  Hits         70347    70347           
  Misses       30550    30550           

Flag	Coverage Δ
cproversmt2	43.38% <ø> (ø)
regression	66.68% <ø> (ø)
unit	32.29% <ø> (ø)

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[martin-cs]

OK.