Ensure weights are correct

[eskimor]

Implement a (generic) fuzzy test to fill up a blocks with random transactions and parameters. Then verify that the resulting block is valid in terms of actual execution time and size.

This test should be generic or easily adjustable and part of any templates. Both system chains and community chains should run this test by default in CI.

[kianenigma]

This, plus #152 would make great tools to ensure it is REALLY HARD to mess your parachains' weights.

[kianenigma]

plus #3581

[kianenigma]

@rockbmb and I talked about this. A quick summary.

There are two weights in any given block.

1. The weight stored in system::BlockWeight storage item. This is computed (excluding the PoV reclaim mechanism) purely based on benchmarking outcome. There is no real measurement of time/proof involved in this. Once the block is done, this number can be seen as a claim: The runtime thinks it has at most consumed this much weight.
2. Then, there is the real weight, which can be calculated by re-executing the very same block, over the reference hardware, in an environment as close to production as possible.

Based on @ggwpez, the building block of this testing is mostly present in benchmark block command which re-executes blocks.

[kianenigma]

plus #5891 is a good step in this direction.