Draft: [backport] [qa] Add test for ArgsManager::GetChainName

Summary

This is a backport of https://github.com/Bitcoin-ABC/bitcoin-abc/commit/b2500c3d81054fc8dd10e35690d69faa6c282c8c See https://reviews.bitcoinabc.org/D5860

• util_SettingsMerge test cleanup

Followup to #15869. Treat "-wallet" as the network-specific argument in test instead of "-server", to make test output clearer and be more consistent with bitcoind. Update embedded hash to match changed output from this.

• Add unit test NextString, ForEachNoDup functions

Remove testcase generating code from util_SettingsMerge so it can be reused in new tests.

The hash value expected in util_SettingsMerge changes as a result of this, but only because the testcases are generated in a different order, not because any cases are added or removed. It is possible to verify this with:

ninja test_bitcoin
ARGS_MERGE_TEST_OUT=new.txt src/test/test_bitcoin --run_test=util_tests/util_ArgsMerge
git checkout HEAD~1
ninja test_bitcoin
SETTINGS_MERGE_TEST_OUT=old.txt src/test/test_bitcoin --run_test=util_tests/util_SettingsMerge
diff -u <(sort old.txt) <(sort new.txt)

The new output is a little more readable, with simpler testcases sorted first.

• Add test for ArgsManager::GetChainName

There was some test coverage previously, but it was limited and didn't test conflicting and negated arguments.

This is a backport of Core PR15988

Test plan

• ninja all check-all