A few notes about the change:
1. I need to initialize the fileclient once, in an init function, so I
don't have access to `testing.T` yet.
2. I want to be able to configure the openapi files that I use
3. We already have a "cache" client that wraps another client, we don't
need to re-implement caching here, one can just do:
`cache.NewClient(openapitest.NewFileClient("some/path"))` to do a cached
client. Or initialize it in an init/global var.
Since there is still some value to use the embedded file, make an
alternative constructor while using fs.FS interface to be able to
manipulate both virtual and disk-based filesystems.
Kubernetes-commit: 29503fd8d45bc2c9438e92936bf4111162529b40
