Add a flag to `compiletest` to control whether it should fail fast or not

**Proposed change:**  Add a `--no-fail-fast` to compiletest which will execute the entire suite regardless of failure.

**Motivation:** This used to be the default behavior but it has changed to always failing fast after #2045. I think both modes are useful, but for CI I would prefer printing all failures in the suite before aborting. The more information the better.