client: don't access UserClientConfigPath or osutil.Homedir unless necessary

Change-Id: I618941204eb561461aab960bde576963296cea87
camput: hermetic client config for test

Change-Id: I3c77b500da2d445851b0d8383a11e7f0942b2ff2
osutil: fail if tests access host paths

Change-Id: I21e20698ebc6b1d3b3499299f6d3dd9ed3031baf