Add test to show that we can fill a block with small Ethereum txns

[notlesh]

What does it do?

Adds a test that should show that we can fill a single block with small Ethereum transctions.

The logic should break down to: 12995000 // 21000 = 618 transactions with some remainder.

Initial results show that we can only include 116 transfers per block.

While I originally wrote this wanting to make sure that the base_extrinsic value in our BlockWeights wasn't causing problems, I suspect that there may be other things going on (such as our recent txpool issues).

[girazoki]

This seems nice, I would merge it with an ignore for now. BTW just make sure to increase the timeout for this test as it is timing out both locally and in the CI.

[notlesh]

I merged master in order to see if this has improved since our recent dependency updates. The test takes about 2 to 4 sec on my machine (the timing mechanism isn't very precise), but does include all of the 618 expected transactions.

[crystalin]

@notlesh I don't think we can rely add this test because we are testing in manual seal and it doesn't have the same rules as our parachain nodes. I was able to push 1200 tx in the block for example (took 6s).

I'd suggest we move it to the para-tests or we close the PR