Increase Reporter extensibility

[tylerFowler]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This change exposes the SubstepResult struct in order to make test.Reporter a fully extensible interface. Currently the only out-of-the-box Reporter implementation invokes its own ReportTestStepFinish method, which does expose this data, but seemingly only to itself (it probably shouldn't be in the main interface for this reason). It relies on a package-private method from ReportTestStep to fill out this data, and the default Executor only invokes that top level method – this prevents users from implementing custom reporters with any level of test-step visibility.

The only way to access this newly public substep data is via the StepResult.GetAllResults method which is already mutex protected so simply exposing this seems safe. To that end only getter methods are added to ensure data mutation is limited.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[linux-foundation-easycla]

• ❌ - login: @tylerFowler / name: Tyler Fowler . The commit (7295a94, 3c3335f) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tylerFowler
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[tylerFowler]

Extra context: I'm creating CL2 tests fully programmatically, I'm skipping the cmd package entirely and loading each step manually, offering myself the ability to implement a custom provider & reporter.