Add `--tests` flag to `spk info`

[jrray]

Our developers are complaining about how our generated CI pipelines contain numerous no-op jobs and one way this happens comes from generating jobs to run spk test on variants that have no tests defined in the recipe.

Currently spk info has a --variants flag to print out what variants are defined in a recipe. We use this in CI to figure out how many different build jobs to create for the CI pipeline.

Similarly, we want to have a --tests flag to print out what tests are defined. Since tests have selectors it is important for the test information to be detailed enough to know what tests would be run for each variant of the recipe.

[rydrman]

From the meeting today:

• possibly spk info --tests-by-variant - we can workshop this but a separate flag feels like the direction