"testdata" is the well-known directory name for files and fixtures
related to a package's test. The toolchain recognizes specifically this
name and ignores it, so it is the proper one to use.

In order to support multiple configuration sources (some managed, some
unmanaged), Retina needs to be able to consider N configuration files
and apply them in an ordered fashion. This makes the GetConfig function
variadic, and uses Viper's MergeInConfig functionality to overlay the
configuration from each subsequent file from the first one.

Trying to figure out a better way to structure this, but since this is a
variadic list, there is now a possibility for zero files (which would be
equivalent to the previous behavior of "" for the file name).

We need to restrict the overlay configuration to a subset of fields, so
that certain configuration may only configure certain fields. This
introduces a FilteredYAML that removes any other fields that it finds so
that *only* those fields that should be allowed to be modified will be.

In order to make it possible for specific fields to be overridden by
user-controllable config files, the GetConfig function must be modified
to support an additional configuration file that can be filtered to
specific fields. This modifies GetConfig in a backwards-compatible way
to support a FilteredConfig type, specifiying additional overlays with
allowable fields from those config overlays.

It turns out that this type is useful outside of this package, which
makes it a good candidate for graduating out of `internal`.

Both of these lints aren't particularly useful, so this just silences
both of them.

In order to merge configurations, we need to be able to write out the
resultant configuration as YAML.

This can be used by variant implementations to decide whether or not TLS
management will be handled by retina or not.