validate: allow config to check for minimum push rate on pads

This change allow tests to check performance of elements by checking the rate at which buffers are pushed on src pads. I re-used most of the logic from fpsdisplaysink to compute the push rate.

We can now uses something like: GST_VALIDATE_CONFIG='core,min-push-rate=60,target-element-factory-name=v4l2src'

The 'start-ts' optionnal field can be used to ignore the rate during the start of the pipeline. This is useful when testing live pipelines where configuring and setting up elements can take some time slowing down the first buffers.