oeqa/oetest.py: use bb logging

oeqa/oetest.py: use bb logging

Plain print has two drawbacks:
- the output is handled by different processes, causing the message
  about found tests to appear randomly after the result of the initial
  tests
- the output is not part of the bitbake console output

Affects image testing with testimage.bbclass.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>