A flag to dune runtest to exit after first test failure

[WardBrian]

Desired Behavior

A --fail-fast (or even better, --max-failures=N, where N=1 means "exit after the first test failure") flag, for circumstances where the important thing is not which tests failed, but simply that some did.

Example

This has several uses, but one would be to make mutation testing a bit easier and more efficient