Fix codegen validation when Runtime APIs are stripped

[tadeohepperle]

fixes #976

[jsdw]

Is it just GitHub or are these lines super indented? :D

[jsdw]

Looks perfect! Do we have any tests to check that the hashes line up when we strip pallets and runtime APIs now from metadata? Let's add one or two just to make sure that the hashes all line up :)

[lexnv]

Suggested change

	/// only hash the provided runtime APIs instead of hashing every runtime API
	/// Only hash the provided runtime APIs instead of hashing every runtime API.

[jsdw]

I had a pass over this to add a few tests for the validation. I also ended up:

• renaming validate_codegen(client) -> Result<(), Error> to is_codegen_valid_for(metadata) -> bool (having it take metadata and return a bool made more sense to me, and then I wanted to tweak the name to match).
• getting rid of the PalletMetadataTestRunner, and upgrading MetadataTestRunner to do validation, too.

So probably somebody that isn't me should review it :)

[niklasad1]

I think set_metadata is better name for this method...

[jsdw]

The build() call is where you pass the actual metadata that the macro will use; this validation_metadata is just the optional metadata you can use to test the is_codegen_valid_for function against :)