tests: allow rpc-tests to get filenames for bitcoind and bitcoin-cli from the environment

This will allow for windows tests to run with bitcoind.exe and bitcoin-cli.exe

qa/rpc-tests/wallet.sh

@@ -8,13 +8,14 @@
 if [ $# -lt 1 ]; then
         echo "Usage: $0 path_to_binaries"
         echo "e.g. $0 ../../src"
+        echo "Env vars BITCOIND and BITCOINCLI may be used to specify the exact binaries used"
         exit 1
 fi
 
 set -f
 
-BITCOIND=${1}/bitcoind
-CLI=${1}/bitcoin-cli
+BITCOIND=${BITCOIND:-${1}/bitcoind}
+CLI=${BITCOINCLI:-${1}/bitcoin-cli}
 
 DIR="${BASH_SOURCE%/*}"
 SENDANDWAIT="${DIR}/send.sh"