tests/service: consolidate setUp/tearDown logic

Consolidate the duplicate code for extension api tests.
This will be also used by servicevm extensions later.

At the same time, a bug in test_extension_firewall.py is also fixed.
double call of super().setUp().

Closes-bug: #1275654
Change-Id: I72f185ef6e5af55729ced51fc0b72fae799d46ac
Merge "tests/service: consolidate setUp/tearDown logic"