Change spec runner to exit with failure for focus: true

[straight-shoota]

This changes the behaviour of the spec runner: In focus mode, the exit code of successful runs will be overriden to 1 to prevent CI from silently running on a subset of tests due to an accidentally commited focus: true.

focus: true is typically used in interactive mode where the print output is most important for the user to understand if the spec run was successful. The failure status code should not have any effect in this use case.

The old behiavour can be regained by setting the environment variable SPEC_FOCUS_NO_FAIL=1. Then the exit code will be success (0) if all focused specs succeed.

Resolves #10989

[HertzDevil]

This default behavior is not immediately obvious to the user, so either the spec runner should print a message upon exit from an otherwise successful run, or crystal spec --help should state this behavior.

[straight-shoota]

focus: true already prints a message:

$ crystal eval 'require "spec"; it(focus: true) {}'
.

Finished in 90 microseconds
1 examples, 0 failures, 0 errors, 0 pending
Only running `focus: true`

[beta-ziliani]

This is somewhat of a breaking change. We should signal that, don't you think?

The change makes sense to me.