tests: adjust extra tests loader to work with nose2
tests: adjust extra tests loader to work with nose2

Nose loader do not provide loader.loadTestsFromTestCase(), use
loader.loadTestsFromNames() instead.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin