Refactor `getTaskRunCreations` to be explicit on getting TaskRuns

[JeromeJu]

The usage of getTaskRunCreations in the pipelineRun reconciler test suite verifies implementation details (API calls) rather than the outputs we care about (TaskRuns themselves), making the tests harder to understand. This should be refactored to not rely on this.

Originally posted by @lbernick in #6135 (comment)

/kind cleanup

[Hussain0520]

Hey @JeromeJu is someone working on this?

[JeromeJu]

Hey @JeromeJu is someone working on this?

@Hussain0520 there is not. Please feel free to self-assign:)

[airycanon]

/assign