test(e2e): add helpers to read/write from/to config

[antoineco]

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• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran make fmt on your commit series before opening this PR;
• Updated relevant documentation.

Description of changes

Part to #370

Allows each test case to have its own ephemeral configuration file, with which it can conveniently interact by reading/writing arbitrary values before executing kraft commands.

The included tests should be self-explanatory.
(I wrote tests for the test framework, and am currently resisting the urge to post a "yo dawg" meme about it.)

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Notes

1. I originally wanted the Write() method to accept a YAML path and a Go value, but figured that avoiding conversions between the Go and YAML type systems was both:

   • simpler to implement, especially due to potentially nested YAML nodes
   • more appropriate for arbitrary manipulations of the YAML config, for example to set multiple values in a single write

   The downside is that test authors need to understand how to use yaml.Filters and how they are executed in a yaml.Pipe.

   As the number of CLI tests grows, we hopefully will see some patterns emerge and decide to revisit that approach.

2. I usually advocate for writing unit tests using Go's bare testing package, and reserve ginkgo for the more complex behaviour-driven things that require setup and teardown.

   In this case though, I used ginkgo for the unit tests as well. The reason is that the helpers included in the test framework embed a few ginkgo functions, so it's easier to test them in the context of ginkgo runs.

[nderjung]

Awesome! Thanks a lot!

Approved-by: Alexander Jung [email protected]