Add some more generated metadata files

[jku]

#1915 adds metadata generation (with stored expected results). We should expand the collection in two directions:

• The current metadata is basically all with default values: Add More variation on the metadata (different key types, delegations, targets, etc)
• Include actual repositories (that are confirmed to work as repositories), possibly with several versions of the same repository: Note that we should be careful not to duplicate the code in RepositorySimulator based updater tests: possibly we could just make RepositorySimulator generated data reproducible and store the results of the RepositorySimulator runs (see the --dump option for related tests)

I'll assign to myself: I'll probably handle the first item and file a new issue on the second but let's see...

[jku]

Pasting in my notes on 1915:

• The key & signer handling could probably be better (why does the generator function need to construct the signers?) but there's no need to polish everything before we know what the code is going to be like...
• The error message could mention that new expected results should only be generated if the change in results was expected
  script only works if run from a specific working directory
• two separate arguments (dump/verify) is weird: that leads to 4 distinct modes but only two of them make sense...

[jku]

Unassigning for now: too busy to handle this right now