Allow returning of test results from Trainer.test

## 🚀 Feature
Currently, `Trainer.test` only returns logged metrics, this is very limiting.

### Motivation

I have a lot of different results which are produced by tests, including figures and sample predictions.
At present, I cannot return them from test since it only allows returning of logged values which it seems must be scalars.

As a workaround I have to separately predict outputs, get targets and then match them together, which is bug-prone.

### Pitch

`Trainer.test` should return output of `Module.test_epoch_end`.

### Alternatives

N/A

### Additional context

N/A

cc @borda @tchaton @justusschock @awaelchli @rohitgr7

Start by tracing the Trainer.test entry point and the Module.test_epoch_end hook to understand how test outputs are currently handled. Review existing Trainer.test tests if available, then verify that returned results include the output from test_epoch_end rather than only logged scalar metrics.

Written by the indexing model from the issue text.