unit/util/string_utils/: Fix compilation on platforms where char is u…

[vmihalko]

…nsigned

Caused following error on ARMv7hl, aarch64, ppc64le, s390x:

FAILED: unit/CMakeFiles/unit.dir/util/string_utils/escape_non_alnum.cpp.o
../unit/util/string_utils/escape_non_alnum.cpp:158:68: error: narrowing conversion of '-128' from 'signed char' to 'char' [-Wnarrowing]
  158 |   CHECK(escape_non_alnum(std::string{static_cast<signed char>(0x80)}) == "_80");
      |

• 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 #6036 (4b819c2) into develop (1c32785) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6036      +/-   ##
===========================================
- Coverage    74.11%   74.02%   -0.09%     
===========================================
  Files         1444     1444              
  Lines       157389   157401      +12     
===========================================
- Hits        116646   116521     -125     
- Misses       40743    40880     +137     

Impacted Files	Coverage Δ
src/goto-instrument/loop_utils.cpp	94.54% <100.00%> (+1.52%)	⬆️
unit/util/string_utils/escape_non_alnum.cpp	100.00% <100.00%> (ø)
src/goto-programs/read_goto_binary.cpp	30.84% <0.00%> (-16.83%)	⬇️
src/ansi-c/c_typecheck_type.cpp	62.96% <0.00%> (-13.43%)	⬇️
src/util/mp_arith.cpp	41.55% <0.00%> (-9.10%)	⬇️
src/util/std_types.cpp	83.95% <0.00%> (-4.94%)	⬇️
src/linking/static_lifetime_init.cpp	95.89% <0.00%> (-2.74%)	⬇️
src/util/json_irep.cpp	92.85% <0.00%> (-1.79%)	⬇️
src/util/unicode.cpp	80.91% <0.00%> (-1.53%)	⬇️
src/util/expr.cpp	74.26% <0.00%> (-1.48%)	⬇️
... and 6 more

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