cmd: charon test mev create block tests

[KaloyanTanev]

We have had tests to test latency towards MEV relay, however, it was a simple ping. Those tests add creation of real block headers.

category: feature
ticket: none

[sonarcloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
9.6% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 11.15385% with 231 lines in your changes missing coverage. Please review.

Project coverage is 57.37%. Comparing base (7ee3282) to head (8f69715).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/testmev.go	11.15%	230 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3378      +/-   ##
==========================================
- Coverage   57.69%   57.37%   -0.32%     
==========================================
  Files         217      217              
  Lines       31619    31874     +255     
==========================================
+ Hits        18241    18288      +47     
- Misses      11495    11701     +206     
- Partials     1883     1885       +2     

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