[JUnit] support launch configuration prototypes

[akurtakov]

One use case is SWT development where I want multiple env variables controlling Gtk/Webkit/SWT/etc. switches to be done at once for a unit test I run.
This is continuation of https://bugs.eclipse.org/bugs/show_bug.cgi?id=536728

[rgrunber]

I think we can salvage https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/157800/ and apply it. My comments from the initial review were addressed.

[jabby]

I have no time to help here, but I'm happy that you opened this issue to continue on the previous work.

[akurtakov]

#1339 enables prototype support for the Generic Test view.