Improve TestYamls and TestExamples

[JeromeJu]

For the current being, there are overlapped tests ie. pipelinerun.yaml in both examples test and yaml test. This is confusing in the case for maintainers to decide whether we should maintain and keep both yamls synced or not. This issue is reported in order to find a way forward for TestYamls and TestExamples, we need to either:

• rewrite TestYamls and TestExamples to avoid testing on repeated yaml cases
• keep the testYamls yaml files and examples synced

cc @lbernick @vdemeester

[JeromeJu]

Related: #6142

[lbernick]

From #3564, it looks like testYamls was introduced to test pipelineruns w/ images built based on the pipelines source, while our examples tests would use the latest versions of the images. Looking at the two yaml tests remaining, they are duplicates of examples tests, and neither of them use ko to build images. So I think we can just remove the yamls tests. @vdemeester to confirm.