Test behavior of list_mbeds
Test behavior of list_mbeds

A new test that catches many corner cases:
 * A target that has a mount point
  * The platform DB is accessed once, with the datum read from the htm
  file on the disk
  * The htm read is called exactly once, with the mount point as the
  argument
  * The return value contains only the data read from the disk, not the
  old data
 * A target that does not have a mount point
  * Is not returned
  * the htm read function is not called
  * the platform db is not consulted

jupe/mbed-ls,jupe/mbed-ls