go/oasis-test-runner/txsource: Decrease MinPoolSize for restarts

[kostko]

When restarts are enabled there can be one less node so we should make sure to
decrease the MinPoolSize in this case.

[codecov]

Codecov Report

Merging #3636 (590f6ff) into master (a235ca4) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3636      +/-   ##
==========================================
- Coverage   67.38%   67.29%   -0.10%     
==========================================
  Files         390      390              
  Lines       36676    36676              
==========================================
- Hits        24715    24680      -35     
- Misses       8505     8542      +37     
+ Partials     3456     3454       -2     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/errors.go	90.00% <0.00%> (-10.00%)	⬇️
go/consensus/tendermint/full/services.go	77.11% <0.00%> (-8.48%)	⬇️
go/common/cbor/codec.go	76.47% <0.00%> (-5.89%)	⬇️
go/storage/mkvs/tree.go	86.95% <0.00%> (-4.35%)	⬇️
go/storage/mkvs/lookup.go	72.04% <0.00%> (-4.31%)	⬇️
go/consensus/tendermint/full/light.go	48.93% <0.00%> (-4.26%)	⬇️
go/storage/mkvs/insert.go	87.67% <0.00%> (-4.11%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	81.13% <0.00%> (-3.78%)	⬇️
go/staking/api/grpc.go	54.81% <0.00%> (-3.71%)	⬇️
go/worker/common/committee/node.go	70.61% <0.00%> (-3.61%)	⬇️
... and 35 more

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