test/py: fix runtest wrapper for pytest 6

The implementation of pytest_runtest_protocol() must call
pytest_runtest_logstart() and pytest_runtest_logfinish(). This appears to
be necessary even in pytest 5.2.1 judging by the default version of
pytest_runtest_protocol(), but evidently some form of code reorganization
in pytest only made this have a practical effect in the newer version. I'd
previously been under the impression that 100% of the required work of
pytest_runtest_protocol() was handled by the fact it called
runtestprotocol() as its implementation. However, it appears that custom
implementations do need to do a little more than this.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>