A gradle build of JUnit tests for all the Java libraries that I commonly encounter.
Gradle was chosen since it comes up so often, that, and I really enjoy how terse and self-documenting
build.gradle can be.
I've found that using Trevor, Git2Go, and GoCoEdit enables me to test any Java library or code I want while on the go, kind of like Sublime Text but for mobile. GoCoEdit integrates with Git2Go so that I can push up complex tests back up to GitHub which Travis CI automatically re-builds per push, so it's like having access to a full native IDE, all on my phone, assuming I've got network access, which is over 99% of the time.