TAC

• This should be a configuration value. A user could want to set this in their configuration.
• The configuration value should be called stop-after. I'm open to other suggestions. I prefer imperative verbs like "stop", but I'll admit it's not obvious what we're stopping after. We could consider something like max-failures or failure-limit.
• Upon reaching the Nth failure, the test runner should immediately print a summary and exit. This will stop any pending tests. We don't want to wait for outstanding tests.
• The summary message should at a minimum indicate that some tests weren't run. Ideally, we would print more information, but the fact that we may not have built all test binaries complicates things. If we really wanted to go for extra credit, we could print the number of tests that we know haven't been executed, and then possibly add the number of tests that we think will be run, once those test binaries were built.
• We should ensure that cargo test --no-run will immediately exit in such scenarios. Presumably it will get a SIGPIPE eventually, but we may want to accelerate the process.

Definition of Done

• CHANGELOG.md updated.
• Documentation in doc/book/head updated.
• Issue is in the correct milestone.