test: don't override BITCOIND and BITCOINCLI if they're set

In rpc-tests.py, don't override BITCOIND and BITCOINCLI if they're
already set. Makes it possible to run the tests with either another tree
or the GUI.

qa/pull-tester/rpc-tests.py

@@ -62,7 +62,9 @@ for arg in sys.argv[1:]:
 
 #Set env vars
 buildDir = BUILDDIR
+if "BITCOIND" not in os.environ:
     os.environ["BITCOIND"] = buildDir + '/src/bitcoind' + EXEEXT
+if "BITCOINCLI" not in os.environ:
     os.environ["BITCOINCLI"] = buildDir + '/src/bitcoin-cli' + EXEEXT
 
 #Disable Windows tests by default