simplify Gradle config

[schmonz]

Users currently have to override Gradle's default parallelization...

tasks.withType<Test> {
    maxParallelForks = 1
}

...because we haven't yet handled combining results from multiple Test Executors.

[schmonz]

To be a reliable and trustworthy addition to a test run, we must (1) always discover the exact same tests a real run would, and (2) always report complete and accurate results of the real run.

If Gradle or Maven or whatnot might be running tests in parallel, then e.g. JUnit might be getting run multiple times, each with some of the tests, and cannot possibly know how to provide (2).

My initial JUnit 5 implementation now feels like the wrong approach, and I don't want to extend it to other frameworks.

To be reliable and trustworthy but also easy to add to existing test configurations, I now think the right approach is to extend whatever drives the test runner and collects the possibly parallel results -- specifically, to extend Gradle, Maven, and other build tools.

[schmonz]

Idea from @truist: another way to deal with parallelization and each little test run being incomplete could be that instead of recording instances of "all green", record instances of "any red" (along with recording whenever any tests have been run). Then if tests have been run recently, but none of them were red, maybe we're okay right now.