"tunnel/tunneld": Prevent credential sharing in shell tests
"tunnel/tunneld": Prevent credential sharing in shell tests

In general, we do not want to simultaneously access VeyronCredential
directories between two processes. This CL fixes the tunneld shell
test so that this is the case.

Change-Id: Idb458f2c419e68b37af40f494d6258108e033c66

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