STEFIL+

Enhancements to the stefil library adding:

• Support for timing of tests, and warnings when tests exceed a designated elapsed time threshold,
• The ability to mark tests as long-running and run test suites with or without long-running tests using the variable stefil+:*long-tests*,
• The addition of an optional test-pre-check argument to defsuite which can be used to toggle the execution of a suite based on the result of a function invocation (e.g., setting test-pre-check to (lambda () (which "foo")) would only run the suite if the foo executable is found on the user's path),
• The addition of a with-retries macro to retry tests with stochastic failures,
• The addition of a run-batch entry point for running a test suite in a non-interactive mode printing a summary of results and returning a truthy value only if all tests pass.