Feat(test-utils/test): BlockContextMocked platform-agnostic + Optimism execution tests

[julio4]

The goal is to make testing utils slightly more platform agnostic to easily test both Ethereum/Optimism low-levels with mocked block context. Example with payload::exec optimism tests included.

for this:

• Add PlatformWithTestnet that every platform needs to define (added complexity to define a new platform but should be straightforward)
• Implementations of BlockContextMocked based on PlatformWithTestnet (+ PayloadBuilderAttributesMocked)
• make rblib_test macro aware of generics and where clauses