veyron/services/mgmt/node: fixing test failure on Jenkins
veyron/services/mgmt/node: fixing test failure on Jenkins

Replacing Mkdir() with MkdirAll() to prevent the former failing if the
test directory to be created already exists (as it is the case on
Jenkins).

Change-Id: I23a034ddb32c1a137a472287c9cee60004113c36

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref