Move Step Validate and ValidateError tests to container_validation_test.go.

[twoGiants]

Changes

Now that Step implements the Validatable interface the tests for the Step validation are moved from task_validation_test.go to container_validation_test.go.

The tests cases which test for a valid Step and for validation errors were extracted from task_validation_test.go:

• TestTaskSpecValidate
• TestTaskSpecValidateError

and moved to new test functions in container_validation_test.go:

• TestStepValidate
• TestStepValidateError

This PR is one of a series to migrate the tests in small batches so that they are easier to review.

More details are in issue #8700.

/kind cleanup

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Release Notes

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/pipeline/v1/task_validation.go	99.1%	99.1%	0.0