go/oasis-node/txsource: Fix event validation in queries workload

[kostko]

There seems to be a bug in the queries workload event validation which fails in case two same events are emitted -- which seems like a perfectly valid thing that can happen now with a single transaction performing runtime message execution emitting events.

Discovered during long-term tests.

[codecov]

Codecov Report

Merging #3593 (1af4d1b) into master (10af4a3) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3593      +/-   ##
==========================================
- Coverage   66.60%   66.58%   -0.02%     
==========================================
  Files         377      377              
  Lines       34562    34562              
==========================================
- Hits        23019    23013       -6     
+ Misses       8228     8227       -1     
- Partials     3315     3322       +7     

Impacted Files	Coverage Δ
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/runtime/host/sandbox/sandbox.go	66.01% <0.00%> (-10.55%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
go/keymanager/client/client.go	78.94% <0.00%> (-4.39%)	⬇️
go/consensus/tendermint/full/services.go	81.35% <0.00%> (-4.24%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	72.41% <0.00%> (-3.45%)	⬇️
go/runtime/nodes/grpc/grpc.go	71.88% <0.00%> (-2.82%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	73.92% <0.00%> (-2.73%)	⬇️
go/storage/database/database.go	68.23% <0.00%> (-2.36%)	⬇️
...consensus/tendermint/apps/scheduler/state/state.go	65.90% <0.00%> (-2.28%)	⬇️
... and 18 more

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