Replace config GUI_LAST_WALLET with CURRENT_WALLET

- CURRENT_WALLET is set when a single wallet is loaded in memory, and it
   remains set after Electrum stops running.
 - If several wallets are loaded at the same time, CURRENT_WALLET is unset,
   and RPCs must specify the wallet explicitly (using --wallet for the CLI)
 - The fallback to 'default_wallet' essentially only applies when
   creating a new wallet file

tests/regtest/regtest.sh

@@ -112,7 +112,7 @@ if [[ $1 == "init" ]]; then
     $agent setconfig --offline test_force_disable_mpp True
     echo "funding $2"
     # note: changing the funding amount affects all tests, as they rely on "wait_for_balance"
-    $bitcoin_cli sendtoaddress $($agent getunusedaddress -o) 1
+    $bitcoin_cli sendtoaddress $($agent getunusedaddress -o -w "/tmp/$2/regtest/wallets/default_wallet") 1
 fi
 
 if [[ $1 == "setconfig" ]]; then