run: return interpreted exit code

[tautschnig]

As already done in other variants of run, decode the exit status using
WEXITSTATUS.

I only noticed this mistake when using --external-sat-solver, which reported weird exit codes (and now reports the expected exit codes instead).

• 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).
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[codecov]

Codecov Report

Merging #6779 (e02f4da) into develop (6127938) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6779   +/-   ##
========================================
  Coverage    76.90%   76.90%           
========================================
  Files         1590     1590           
  Lines       183970   183971    +1     
========================================
+ Hits        141485   141486    +1     
  Misses       42485    42485           

Impacted Files	Coverage Δ
src/util/run.cpp	75.63% <100.00%> (+0.20%)	⬆️

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