Add a new base class called `TestBase` to our test-suite. This allows
sharing common code between our tests without requiring them to import
each other. Furthermore, it paves the way towards executing all our
tests as part of the `unittest` framework, including pylint and others.
For now, this adds the following features to `TestBase`:
* Common test-guards that are shared between our tests, like
`can_modify_immutable()` or `have_rpm_ostree()`.
* Accessors to the test-checkout. This is `have_test_checkout()` to
check whether the running test has a repository checkout, and
`locate_test_checkout()` to get a path to the repository checkout.
This will allow us to put pylint and friends into the unittest
framework, guard them properly, and still allow running the tests
from a global install which might not have access to a checkout.
For now, we always assume we run from a checkout.
* Accessors to test-data. If we start installing tests as a module
into the system, we cannot bundle test-data together with code.
Therefore, two accessors `have_test_data()` and `locate_test_data()`
are implemented to guard access to test data. If a checkout is
available, it will be used to locate test-data.
In the future, we want to be able to pass a separate path to the
test-data, thus allowing us to install tests into a system.
