It's possible to have the java command line tool without a runtime

installed, which prevents anything from executing. The Java support

wasn't built for this, incorrectly thinking that dependencies were met,

but failing to run.
This enhances JavaToolMixin to first run java -version, checking the

error code, and caching a _has_java_runtime class attribute, for

sharing across tools. Helper methods are available for unit tests to set

or clear this state, for testing.
The PMD tool now uses JavaToolMixin, in order to take advantage of

this. Unlike other Java-based tools, PMD runs a wrapper script, rather

than running java directly. This required some changes in

JavaToolMixin to make fewer assumptions about its use.
And finally, the PMD tool had some code that assumed Unicode strings,

but broke with byte strings. The simulation tests gave it Unicode

strings, but it didn't instrument correctly with byte strings. The tests

have now been updated.

All relevant unit tests pass without java, with java but no runtime,

and with a runtime.