validate: allow config to check for minimum buffer frequency on pads

validate: allow config to check for minimum buffer frequency on pads

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

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

The 'buffer-frequency-start' optional field can be used to ignore the
frequency 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.