Migrate the remaining (standalone) test/ext tests to pytest

Migrate the remaining (standalone) test/ext tests to pytest

Enable the test/ext/conftest.py collector to allow pytest to handle
all of the test/ext standalone tests too.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: show the pytest invocations; switch to use
 test/ext/conftest.py collector; use the release mark in check and
 distcheck; augment commit message; augment the information in
 HACKING]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>