[Coretime chain] Add high assignment count mitigation to testnets (#6022)

Fixed in Polkadot and Kusama in
https://github.com/polkadot-fellows/runtimes/pull/434 and this PR just
adds to testnets.

We can handle a maximum of 28 assignments inside one XCM, while it's
possible to have 80 (if a region is interlaced 79 times).
This can be chunked on the coretime chain side but currently the
scheduler on the relay makes assumptions that means we can't send more
than one chunk for a given core.

This just truncates the additional assignments until we can extend the
relay to support this. This means that the first 27 assignments are
taken, the final 28th is used to pad with idle to complete the mask (the
relay also assumes that every schedule is complete). Any other
assignments are dropped.

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Co-authored-by: Kian Paimani <[email protected]>