Test that all the explorer still work, both for transactions (History
tab) and for address URLs (Address tab).
Edit `~/.electrum-abc/config`, set `"block_explorer": "ViaWallet"`,
bump the version in version.py to trigger the update,
check that the "CoinEx" explorer is selected in the configuration menu.
Check that the line in `config` was
updated to `"block_explorer": "CoinEx"`