runtime/dispatcher: Break recv loop on abort request

[ptrus]

Ideally this should be caught by:

oasis-core/go/runtime/host/tests/tester.go

Lines 180 to 196 in b7b2c42

	select {
	case ev := <-evCh:
	require.NotNil(ev.Started, "should have received a successful start event")
	case <-time.After(recvTimeout):
	t.Fatalf("Failed to receive event")
	}
	// Trigger a non-force abort (runtime should not be restarted).
	err = r.Abort(context.Background(), false)
	require.NoError(err, "Abort(force=false)")
	// There should be no stop event.
	select {
	case ev := <-evCh:
	require.Nil(ev.Stopped, "unexpected stop event")
	case <-time.After(recvTimeout):
	}

But the problem is that the Abort request makes it to the dispatcher before the first iteration of the dispatch loop, therefore the request is handled before the dispatcher is stuck in the recv loop.
Updated the test to do another abort which also handles the case where dispatcher is stuck in the recv loop.

[codecov]

Codecov Report

Merging #3023 into master will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3023      +/-   ##
==========================================
- Coverage   68.62%   68.45%   -0.17%     
==========================================
  Files         367      367              
  Lines       36130    36130              
==========================================
- Hits        24795    24734      -61     
- Misses       8165     8207      +42     
- Partials     3170     3189      +19     

Impacted Files	Coverage Δ
go/worker/compute/executor/committee/state.go	74.07% <0.00%> (-25.93%)	⬇️
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
go/worker/compute/executor/committee/node.go	61.48% <0.00%> (-5.70%)	⬇️
go/runtime/committee/nodes.go	80.39% <0.00%> (-3.93%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	77.58% <0.00%> (-3.45%)	⬇️
go/storage/metrics.go	78.26% <0.00%> (-3.27%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	61.26% <0.00%> (-2.11%)	⬇️
go/storage/mkvs/lookup.go	73.46% <0.00%> (-2.05%)	⬇️
go/runtime/client/watcher.go	73.84% <0.00%> (-1.54%)	⬇️
... and 17 more

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