tests fail due to large amount of output with highish tests.iters

[msokolov]

Description

I've noticed that some tests fail when tests.iters is set to a highish value (like 20) because stdout grows above the allowed threshold. I assume this is something we'd want to fix, but I'm not clear how to go about it exactly. Some tests I've seen fail are: TestDemo.testIndexSearch (IndexFiles outputs the names of files as they are added to the index); TestIndexWriter.testDocValuesMixedSkippingIndex (prints a stack trace, I think it's expected?); TestTransactions prints out an iteration count. We could work on reducing the amount of printing, but I wonder if the test framework should ideally multiply the acceptable stdout threshold by the iters?

Version and environment details

all versions, probably