go/oasis-test-runner/fixture: support configuring runtime messages

[ptrus]

Adds support for configuring roothash max messages in network fixtures.

[codecov]

Codecov Report

Merging #4769 (d34cb93) into master (4e50402) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4769      +/-   ##
==========================================
- Coverage   66.56%   66.53%   -0.03%     
==========================================
  Files         447      447              
  Lines       50065    50065              
==========================================
- Hits        33328    33313      -15     
- Misses      12583    12587       +4     
- Partials     4154     4165      +11     

Impacted Files	Coverage Δ
go/oasis-node/cmd/genesis/genesis.go	51.95% <100.00%> (ø)
go/keymanager/api/grpc.go	18.42% <0.00%> (-21.06%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/worker/storage/committee/utils.go	92.30% <0.00%> (-7.70%)	⬇️
go/worker/common/p2p/dispatch.go	71.52% <0.00%> (-5.56%)	⬇️
go/worker/storage/p2p/sync/client.go	86.95% <0.00%> (-4.35%)	⬇️
go/consensus/tendermint/apps/registry/query.go	78.26% <0.00%> (-4.35%)	⬇️
go/oasis-node/cmd/ias/auth_registry.go	64.10% <0.00%> (-4.28%)	⬇️
go/consensus/tendermint/registry/registry.go	66.18% <0.00%> (-3.87%)	⬇️
... and 24 more

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