Testing updates

* Use in-process alternatives to `jar` and `javac` (faster and fixes errors when JDK tools are not in the system path)
* Suppress nonsensical `ARGUMENT_COUNT_MISMATCH` with the following form:

    > passed 2 arg(s) to template /foo with 2 declared arg(s)

* Fix test failures

* Allow exceptions in process execution to propagate to the caller so test failures are properly reported