test: fix rpcauth test failure when test/config.ini is missing

[lustsazeus-lab]

Summary

• make test/util/rpcauth-test.py runnable from a fresh source checkout where test/config.ini has not been generated yet
• keep current behavior when test/config.ini exists
• add fallback import path to share/rpcauth/rpcauth.py for local/direct unit test runs

Why

The current test hard-fails with FileNotFoundError when run directly in a clean checkout:

python3 test/util/rpcauth-test.py
FileNotFoundError: .../test/config.ini

This change fixes that path so contributors can run the rpcauth unit test without doing a full config generation step first.

Validation

• python3 test/util/rpcauth-test.py ✅ (3 tests passed)
• python3 -m py_compile test/util/rpcauth-test.py ✅

Fixes #32

@bountydotnew submit

[lustsazeus-lab]

Friendly ping on this PR — it has been sitting open for a while and is currently mergeable (CLEAN).

If anything needs to be adjusted on my side, I can update it quickly. Otherwise, I'd appreciate a review when you have a moment.