tests: partially convert to pytest

tests: partially convert to pytest

Convert all the python-based library unit tests to pytest,
and add a pytest.ini to only look in the test/ directory.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: run pytest outside wvtest check wrapper so the
 output is visible; restore some of buptest.py and wvtest.py so the
 other tests can still run; rename pytest shim to wvpytest to
 distinguish from ./wvtest.py.; add ./pytest to centralize common
 options; switch .wvtest import to wvtest now that lib/test is a proper
 module dir; narrow pytest.ini test dirs to test/int and test/ext; move
 sort-z to test/bin; fix some dependencies in dev/prep-for-*]