go/roothash/reindexBlocks: return latest known round if no new rounds are reindexed

[ptrus]

This fixes a case where a storage node would not register if restarted while synced and there were no new runtime rounds (e.g. the runtime is suspended).

Also updates the runtime-dynamic scenario to test this case.

[codecov]

Codecov Report

Merging #3791 (5bb676e) into master (a1120a7) will decrease coverage by 0.16%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #3791      +/-   ##
==========================================
- Coverage   67.27%   67.10%   -0.17%     
==========================================
  Files         403      403              
  Lines       40904    40912       +8     
==========================================
- Hits        27518    27454      -64     
- Misses       9498     9567      +69     
- Partials     3888     3891       +3     

Impacted Files	Coverage Δ
go/consensus/tendermint/roothash/roothash.go	67.16% <75.00%> (-6.19%)	⬇️
go/runtime/host/sandbox/sandbox.go	69.53% <0.00%> (-12.11%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	69.23% <0.00%> (-7.70%)	⬇️
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-6.55%)	⬇️
go/worker/common/committee/runtime_host.go	64.28% <0.00%> (-6.25%)	⬇️
go/consensus/api/submission.go	67.10% <0.00%> (-5.27%)	⬇️
go/storage/api/root_cache.go	71.11% <0.00%> (-4.45%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	73.49% <0.00%> (-2.82%)	⬇️
... and 27 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1120a7...5bb676e. Read the comment docs.