bts_nanobts: Fix pwsup_list increasing its size every new test

variables under class section are shared among all instances, and are
used unless the specific class instance attribute is set. As a result,
the same pwsup_list object was being re-used.

Take the chance to do the same with the _pcu var, since we want same
behavior for it.

See for reference: https://stackoverflow.com/a/1716730

Fixes: OS#3456
Change-Id: I3f6a9d64b35aec33430e90872f41c3d046653ce6
1 file changed