Testing: Add integration testsuite

[SchoolGuy]

Fixes #51

This adds integration testing with the different versions of Cobbler. Workflow and scripts have been copied from the Terraform provider.

The goal of this PR is to rebuild the current Python testsuite that the backend has with the built-in CLI.

To make our code testable we need to be able to redirect the output to something that is not stdout. This can be achieved through cobras cmd.OutOrStdout() function.

Furthermore, we need to make generateCobblerClient() return an error and have that handled by cobras Command.RunE() method that we are using to execute our logic.

Lastly, we need to build ourselves a method to generate new instances of the commands because cmd.Execute() can only be called once.

[SchoolGuy]

Once GitHub Actions have recovered I will add more tests. Locally the one test I have added is failing successfully.

[SchoolGuy]

Test setup in the CI with the different versions of Cobbler works as expected.

[SchoolGuy]

The CLI is now prepared to be tested. I will start writing tests tomorrow.

[SchoolGuy]

We are affected of issue golang/go#33976 in our tests for the version command. The change is not released in any Go version at the moment. As such we need to test for the empty source in that test.

[SchoolGuy]

Locally the only broken tests with 3.3.6 are related to Image and Systems. All individual tests of both item types fail with the same issues correspondingly. Furthermore, it seems that empty interfaces create XML issues with the system item type.

PR cobbler/cobblerclient#75 in the client will act as a fixup PR for locally fixed issues in the client.

[SchoolGuy]

Image tests are locally passing now. The system tests are refusing to work because the sync tests in combination with the system add test is causing dhcpd to exit abnormally, thus causing an issue with the post change service restart trigger.

[SchoolGuy]

All tests are now passing locally. The issue was solved by adding a forced sleep for the duration of a second.

[SchoolGuy]

An update to 0.5.5 with bugfixes that are needed will cause the tests to pass.

Edit: The client version 0.5.5 will be released after cobbler/cobblerclient#75 was merged.

[SchoolGuy]

The Codacy Bash script errors are partly correct but I would need to fix that on the Terraform Provider as well. As none of those issues affect the delivered binary, I would like to skip those for now.

[tiltingpenguin]

The design of the tests looks sound to me, though for the future I would like to see some negative tests as well. But as long as the tests pass this is ok for me. I just found some nitpicks