Option for a non-zero exit status for a failed unknown test

I recently ran into the issue described here: https://github.com/gotestyourself/gotestsum/issues/141
Basically, test2json couldn't parse a test result (because I used fmt.Printf() without a linebreak), which caused gotestsum to report the test as `failed (unknown)`.

This seems perfectly fine behavior, but I'd really like the tests to fail if a test returns a `failed (unknown)` status, so we can improve the code. Currently however gotestsum still exists with exit code 0 if this occurs.

Start by reading the linked gotestsum issue 141 and tracing where gotestsum determines its exit status after reporting a failed (unknown) test. The work is done when an option makes that status return a non-zero exit code while preserving the current behavior by default.

Written by the indexing model from the issue text.