adder-collator: add velocity measurement and make elastic scaling test more robust (#4016)

Improves `adder-collator` to also compute the parachain velocity. The
velocity is defined as number of parachain blocks progressing per relay
chain block.

In this test we're asserting that the elastic parachain always
progresses by 3 blocks per RCB, while the non-elastic parachain
progresses normally - 1 block per RCB.

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Signed-off-by: Andrei Sandu <[email protected]>
Co-authored-by: ordian <[email protected]>