Merge bitcoin#24375: Do not use LocalTestingSetup in getarg_tests test file.

5d7f225 Do not use `LocalTestingSetup` in getarg_tests test file. (Kiminuo)

Pull request description:

  Avoid using a test fixture in getarg_tests for better readability. Change was implemented by _kiminuo_ and posted bitcoin#24306 (comment)

ACKs for top commit:
  kiminuo:
    ACK 5d7f225

Tree-SHA512: 0fd98622010e6923e91c66447a1d0861bf344a65d86a313dff7d428c089b1740a25f699327f6ed4c163255f270bcbd4f7be962bb551862214f9b9e395d40df04

Author: MarcoFalke