Explore adding test coverage for `exec`

[lread]

Problem

We don't have test coverage for babashka.process/exec.

Technical

The exec fn can only be used from an executable created by GraalVM native-image.

To think about

We don't want to replace our test runner process, ya?

Next Steps

I can come up with something if you like.

[borkdude]

I'd also be fine if we just did these tests as babashka's test suite.

[lread]

Another column for the grid!

To sum up a chat we had on slack:

1. it is probably ideal to do the native-image exec testing here in babashka/process
2. but it could be more practical/easier to do them in babashka.

I'll look into 1, we can always fall back to 2.