Add more comprehensive tests that work with online and offline tests

[lukpueh]

Description of issue or feature request:
Restructure existing online and offline tests and add more comprehensive test scenarios that may be used for both.

Current behavior:
Currently there are two ways of testing the apt transport:

1. Online test using real remote rebuilders and real debian apt and apt http transport (see tests/Dockerfile)
2. Offline test that mocks rebuilders, apt and apt http transport (see tests/test_intoto.py)

Each currently performs one specific scenario of package installation using apt, which triggers in-toto verification with the proposed intoto transport.

Expected behavior:

• Add more comprehensive test cases, i.e. passing/failing in-toto verification (with different combinations of configuration values).

• Adapt online and (above all) offline test setup so that both can run the same test cases.

[lukpueh]

For now #18 sufficiently addresses this issue (it brings coverage with offline mock installation and unittests up to 99%).

Using the same test scenarios with online tests would be nice to have but is not a priority. We can revisit the issue if the need arises.