tests: py: any: Make tests more generic by using other interfaces

tests: py: any: Make tests more generic by using other interfaces

Some tests use hard coded interface names and interface indexes.
This commit removes these cases by exchanging "eth0" with "dummy0" and
"lo" (depending on the test) in all ifname tests and by using "lo"
instead of "eth0" in all interface index tests (because we can assume
"lo" ifindex is 1).

Signed-off-by: Manuel Johannes Messner <manuel.johannes.messner@hs-furtwangen.de>
Signed-off-by: Florian Westphal <fw@strlen.de>