python3: Add ptest support

python3: Add ptest support

A run-ptest script written in python is added which defines a new TestRunner
subclass which prints test results in the required ptest format and then
executes python's built-in testsuite using this new TestRunner subclass.

The built-in testsuite is included in the python standard library and we ensure
we have the complete library by adding ${PN}-modules and ${PN}-misc to the
RDEPENDS for the ptest package. We also require libgcc for pthread support.

Tested on qemux86. Several tests fail and many skip; further investigation
should be done to check that the current status is acceptable. In addition, the
testsuite causes an out-of-memory crash when qemu is configured with 256 MB of
RAM. The testsuite completes without crashing with 1 GB RAM.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>