A more convenient interface to get a throwaway empty credentials source
for use in tests, which doesn't interact at all with the real CLI
configuration directory.
This new implementation is not yet used, but should eventually replace the
technique of composing together various types from the svchost/auth
package, since our requirements are now complex enough that they're more
straightforward to express in direct code within a single type than as
a composition of the building blocks in the svchost/auth package.