colexec: TestScanBatchSize flaked

See https://teamcity.cockroachdb.com/viewLog.html?tab=buildLog&logTab=tree&filter=debug&expand=all&buildId=2811705&_focus=203528


```
    vectorized_batch_size_test.go:67: 
        	Error Trace:	vectorized_batch_size_test.go:67
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 9
        	Test:       	TestScanBatchSize
```