Make sinter's test_main_collect less flaky

[Strilanc]

data = ExistingData.from_file(d / "out.csv").data
            assert len(data) == 3
            for k, v in data.items():
                assert v.discards == 0
                assert v.errors <= 50
>               assert v.shots >= 1000
E               AssertionError: assert 948 >= 1000
E                +  where 948 = TaskStats(strong_id='69f2036fdc2ccc88b1da0138bb546f1582c230618d3d2083e9eaecc[60](https://github.com/quantumlib/Stim/actions/runs/4113718436/jobs/7100582378#step:9:61)c5e8800', decoder='pymatching', json_metadata={'path': '/tmp/tmpccue__43/3.stim'}, shots=948, errors=12, discards=0, seconds=0.3[61](https://github.com/quantumlib/Stim/actions/runs/4113718436/jobs/7100582378#step:9:62)).shots