## What changes were proposed in this pull request?

This makes microbenchmarks run for at least 2 seconds by default, to allow some time for jit compilation to kick in.

## How was this patch tested?

Tested manually with existing microbenchmarks. This change is backwards compatible in that existing microbenchmarks which specified numIters per-case will still run exactly that number of iterations. Microbenchmarks which previously overrode defaultNumIters now override minNumIters.

cc hvanhovell

Author: Eric Liang <ekl@databricks.com>
Author: Eric Liang <ekhliang@gmail.com>

Closes #13472 from ericl/spark-15735.