test(third PR): add reconcile child pipeline tests

The test setup is a parent pipeline with one or more embedded
child/grandchild pipelines using the `PipelineSpec` (alpha) field. It
follows the given-when-then test flow arrangement.

The pipeline manifests yaml definitions use variables for every field
which is validated. Multiple helper functions were created equal to
reconciliation unit tests for TaskRuns/CustomRuns:
- `testing.OnePipelineInPipeline`
- `testing.TwoPipelinesInPipelineMixedTasks`
- `testing.NestedPipelinesInPipeline`
- `parse.MustParseChildPipelineRunWithObjectMeta`
- `reconcileOncePinP`
- `validatePinP`
- `getChildPipelineRunsForPipelineRun`
- `getChildPipelineRunByName`
- `validateChildPipelineRunCount`
- `getChildPipelineRunByName`
- `testing.VerifyChildPipelineRunStatusesCount`
- `testing.VerifyChildPipelineRunStatusesNames`
- `testing.childPipelineRunWithObjectMeta`

Test data factory functions were put in the `testing` package in the
`factory.go` file.

The test validates:
- the status and condition of the parent PipelineRuns which should
trigger the creation of the child PipelineRuns,
- the actual created child/grandchild PipelineRuns if they have the correct
metadata i.e. name, owner reference, etc. and the embedded pipelines
from the `PipelineSpec` fields in the pipeline tasks of the parent
pipeline.

Similar checks are performed in `TestReconcile` for `TaskRun` and in
`TestReconcile_V1Beta1CustomTask` for `CustomTasks`.

Add annotations/label propagation tests:
- `TestReconcile_PropagateLabelsAndAnnotationsToChild`
- `TestReconcile_ChildPipelineRunHasDefaultLabels`

Aadd child pipeline creation error tests. New table driven test
`TestReconcile_ChildPipelineRunCreationError` and test framework
additions:
- `type clientErrors`
- `reconcileWithError`

`reconcileWithError` can be reused in every test with fake client error
returns i.e. where `Pipeline.PrependReactor` is used.

Add e2e child pipeline tests. The test setup is a parent pipeline with
one or more embedded child/grandchild pipelines using the `PipelineSpec`
(alpha) field equal to the reconciliation unit tests. The e2e tests also
follow the Given-When-Then test structure.

The pipeline manifests yaml definitions use variables for every field
which is validated. Multiple helper functions were created equal to
PipelineRun e2e tests:
- `setupPinP`
- `tearDownOptDump`
- `createKindsMap`
- `assertPinP`
- `createResourcesAndWaitForPipelineRun`
- `checkLabelPropagationToChildPipelineRun`
- `checkAnnotationPropagationToChildPipelineRun`
- `assertEvents`

The tests validate:
- parent PipelineRun creation,
- child PipelineRun creation,
- successful finish of all resources,
- correct label and annotation propagation,
- amount of events published.

Similar checks are performed in
`TestPipelineRun|TestPipelineRunStatusSpec|...` for `TaskRun` and in
`TestCustomTask` for `CustomTask`.

Issues tektoncd#8760, tektoncd#7166

Signed-off-by: Stanislav Jakuschevskij <stas@two-giants.com>

Author: twoGiants