testsdk.bbclass: check python module testtools and subunit

testsdk.bbclass: check python module testtools and subunit

The testtools or subunit is not part of python's standard library, so check them
before use.

Fixed when they are not installed on host:
$ bitbake core-image-minimal -ctestsdk
Exception: ImportError: No module named 'testtools'

Now it can run with a warning:
WARNING: core-image-minimal-1.0-r0 do_testsdk: Failed to import testtools or subunit, the testcases will run serially

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>