test: Move hex0x() to utils.hpp

[chfast]

hex0x() needs nothing from statetest.hpp, which pulls in nlohmann/json and the whole state model. Moving it to utils.hpp, next to the other small conversions, lets lighter headers use it.

[codspeed-hq]

Merging this PR will not alter performance

✅ 129 untouched benchmarks

---

_{Comparing test/hex0x-to-utils (2a85d90) with master (c8d6705)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.72%. Comparing base (c8d6705) to head (2a85d90).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1675   +/-   ##
=======================================
  Coverage   97.72%   97.72%           
=======================================
  Files         171      171           
  Lines       15666    15666           
  Branches     3625     3625           
=======================================
  Hits        15310    15310           
  Misses        269      269           
  Partials       87       87           

Flag	Coverage Δ
eest-develop	88.60% <0.00%> (ø)
eest-develop-gmp	26.51% <0.00%> (ø)
eest-legacy	17.09% <0.00%> (ø)
eest-libsecp256k1	28.78% <0.00%> (ø)
eest-stable	88.60% <0.00%> (ø)
evmone-unittests	93.44% <100.00%> (ø)

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Components	Coverage Δ
core	96.11% <ø> (ø)
tooling	91.96% <100.00%> (ø)
tests	99.80% <ø> (ø)

Files with missing lines	Coverage Δ
test/utils/statetest.hpp	76.92% <ø> (-3.08%)	⬇️
test/utils/utils.hpp	100.00% <100.00%> (ø)

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