Add tests for flux trace command

Add tests for flux trace command that fake out the kubernetes client,
load objects from a yaml file and create them in the client, and
assert on the output of the trace command to an expected golden file.

This is a follow up from the suggestions in PR https://github.com/fluxcd/flux2/pull/1626

 which suggested that additional
testing would be helpful. This test approach is modeled after the helm command tests.

This required some changes to the kubernetes client setup to make it
possible to use a fake. If we agree this pattern makes sense, it can be
applied to other commands.

Signed-off-by: Allen Porter <allen@thebends.org>