tests: Add wait_for function

This function makes it possible to wait for a specified condition in a test,
some examples:

  # Wait up to 1 minute for a file to be created
  assert_success wait_for 'ls | wc -l | grep "[1-9]"' '1 minute'

  # Wait up to 30 seconds for 8 chunks to be created
  wait_for '(( $(mfsdirinfo dir | grep chunks: | awk "{print \$2"}) >= 8 ))' '30 seconds'

Change-Id: I29d1892eb8c6aa84da263ee33738c57fcfe3ce91
copyright: Update author list

Change-Id: I79a36080cab26fad413d7a61f32cc3d7b46ba080