Use random seed on synthetic data in mag tests

[santisoler]

Summary

Make use of the random_seed argument of the make_synthetic_data method in magnetic tests. Remove the lines that set a global np.random.seed in those tests.

PR Checklist

• If this is a work in progress PR, set as a Draft PR
• Linted my code according to the style guides.
• Added tests to verify changes to the code.
• Added necessary documentation to any new functions/classes following the
  expect style.
• Marked as ready for review (if this is was a draft PR), and converted
  to a Pull Request
• Tagged @simpeg/simpeg-developers when ready for review.

Reference issue

Part of the solution to #1289

What does this implement/fix?

Additional information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.33%. Comparing base (ae63c36) to head (b3d44a1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1457   +/-   ##
=======================================
  Coverage   82.33%   82.33%           
=======================================
  Files         171      171           
  Lines       26019    26019           
=======================================
  Hits        21422    21422           
  Misses       4597     4597           

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