Force the user to explicitly specify a base URL for getting trust
metadata, rather than constructing one from one of the "defaults"
channel.  The practical effect is to squash warnings about missing
signature verification for popular channels (repo.anaconda.com,
conda-forge) that we know currently do not have metadata signatures.

Prevents certain errors when metadata verification is enabled while the
package metadata cache is still active.

Check that this config option actually gets loaded; rename previous test
to avoid name collision and better describe what it does.