Set outputs for plugin-test action

As a developer I would like to have the output from the plugin test command to validate my test results as part of the workflow. The improvement to the plugin action could be like:

```yaml
...
outputs:
stdout:
description: "An array of stdout lines the test command"
stderr:
description: "An array of stderr of the test command"
debug:
description: "An array of debug lines of the test command"
```

The output essentially are the lines from the `exec.exec(...)`

Start at the plugin-test action entry point and its exec.exec(...) call; inspect how stdout, stderr, and debug lines are currently captured. Define the three action outputs shown in the issue and verify that they expose the test command’s lines in a workflow.

Written by the indexing model from the issue text.