Turn off light client tests for feature specs

[hwwhww]

Issue

The current test suite requires light client specs by default, even though most feature specs do not focus on light client design.

IMO unless a feature specifically pertains to light client design, these light client specs within feature specs serve little purpose.

How did I fix it

• Create a LIGHT_CLIENT_TESTING_FORKS setting and set it from Altair to Deneb.
• Use with_light_client decorator for the light client tests
• Delete EIP-6110 light client specs

It will help minimize the overhead associated with adding new feature specs.

[mkalinin]

I am pretty much in favour of this proposal! It allows to get rid of unnecessary complexity required to define a new feature

[djrtwo]

I support this!

light client spces should probably be built for a particular Fork or as a particular feature itself (EIP). Otherwise, it's just a ton of typing and boilerplate work for no gain