[TEP-0100] Prepare for testing of minimal status implementation

This change is to minimize the size of the actual implementation. We need to change a number of
tests in `pkg/reconciler/pipelinerun/pipelinerun_test.go` to be table-based, so that we can test
behavior under each possible value for the new `embedded-status` feature flag. Here, we just
modify the relevant tests to be table-based and use a parameterized helper function for the actual
execution, with a test case for each value.

It also adds helper functions for checking the relevant fields in the `PipelineRun` status, which
are hard-coded to always handle the current, "full" embedded status approach.

This also splits out `TestUpdatePipelineRunStatusFromTaskRuns` and `TestUpdatePipelineRunStatusFromRuns`
into a separate file, `pipelinerun_updatestatus_test.go`. When the TEP-0100 implementation lands,
this will also contain additional tests for updating via child references. Splitting like this helps
keep `pipelinerun_test.go` from getting even more bloated than it is currently.

Until the implementation, these table-based tests are purely duplicative - they're going to run the
same and check the same things for any value of `embedded-status`, but the implementation PR will be
cleaner, only adding the implementation and changing the helper functions to take all of the
possible "embedded-status" values into account. The changes which need to be made in the
implementation PR are all marked with `// TODO(abayer): ...` in `pipelinerun_test.go` and
`pipelinerun_updatestatus_test.go`.

Signed-off-by: Andrew Bayer <andrew.bayer@gmail.com>