Allow JVM options to be passed for JUnit testing via -DargLine=...

Sometimes, we need to pass JVM options (such as -Djava.awt.headless=true
or -verbose:class) to the JVM running the unit tests. Unfortunately,
the maven-surefire plugin does not seem to expose a command-line
interface, so simulate it by setting argLine to ${argLine}.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'surefire-argline'

This adds support to pass options to the JVM running the unit tests using
something like:

	mvn -DargLine=-verbose:class test

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>