With the unit test framework, we define special methods, like setUp()
and test_*(). This is documented, but not obvious.

Previously, TestNmClient was the base class for our tests classes, and
it provided some functionality (and state). It was utterly confusing how
pieces fit together.

Instead, move the state to a new class NMTestContext(). That contains
most of the code from TestNmClient. Drop TestNmClient and let the test
classes directly descend from unittest.TestCase.

The difference is, when you now look at a certain test (test_001()), you
can easier understand which code runs when. First, the test class has a
setUp() method which runs, but that method is now trivial without extra
context. Second, there is the @nm_test attribute that wraps the
function. But that's it. It's all at one place, and we delegate instead
of inherit.

The goal is to make the base class TestNmClient smaller because it
contains a lot of state, and with inheritance it's hard to follow what
happens.

These two are stateless method, and even not only work for nmcli, but also for
nm-cloud-setup. Move to the Util class, as static methods.

They can both be used as decorators and called plainly.

Static, state-less functions are great. Keep them separate from TestNmClient which
has lots of state.

Don't rely on resources provided by mock metadata server by default,
create the from within the test instead.

This allows for more flexibility, but the locality of the test fixture
relative to the tests makes the test more legible.

We're going to reuse the setup for tests of other cloud providers.