[RFC] reporegistry: embed the test repo in the go binary for easier use

[mvo5]

[draft as this needs splitting into two commits, more tests and is also a bit of a discussion starter if it's a good idea, it will also make the binaries bigger, e.g. the "build" binary increases from 25MB -> 28MB (stripped), otoh only things that "import testdata" will be affected so should not be too bad]

Running cmd/build/build currently requires running it from a git checkout so that it can find the required repository files.

This commit embeds them into the binary via "go:embed" to make it easier to use build on it's own.

This will make working with otk to generate reference images easier.

[achilleas-k]

I think adding a --repositories (or --repos to save some typing ^_^) would be a nicer approach to this. There are many cases where I would like to generate manifests or build an image with the same definitions but different repositories. For example, if a repository snapshot update causes errors, the investigation would be much easier if I could run the same binary (same image definitions) against two separate repository configurations, without needing to recompile.

[mvo5]

Closing as something we currently do not want.