When I implement new features using TDD, I often use the shortcut Shift + Alt + D T to run only the jUnit test case I am currently working on (and not the whole tes
Windows → Preferences → JUnit has an option to add -ea every time a new launch configuration is created. It adds the -ea option to the Debug Configuration as well.
The full text next to a check box is:
Add
-eato VM arguments when creating a new JUnit launch configuration
Only way I can see to do it is to contribute a new launch configuration, this is not a trivial task but not too hard if you have PDE experience.
There is an eclipse.org article (see "Creating a Launch Configuration") that describes how to construct configurations and launch them.
I guess you could copy the JUnit implementation (see the org.eclipse.debug.core.launchConfigurationTypes extension in org.eclipse.jdt.junit) and add in your chosen defaults to that config. You'd then need to declare a key binding to launch it and package the plugin to your install.
I think you can set this option by editing the definition of the JRE under Preferences → Java → Installed JREs → Select the JRE in question and edit the default VM arguments