tests: Add assert_awk_finds[_no] assertions

These assertions help users to verify contents of long strings
in various tests. Eg.

  x=$(come-command-which-prints-a-lot-of-lines)
  # Assert that there is a line which has '5' as the second field
  assert_awk_finds '$2 == 5' "$x"
  # Assert that there is a line which begins with 'entry'
  # and has '5' as the second field
  assert_awk_finds '/^entry/ && $2 == 5'

Change-Id: I46004a553a77b38d2a626078bb73d77e5bcd7e01