Fixes https://github.com/eclipse-pde/eclipse.pde/issues/179

For newly created JUnit Plug-in Test launches the automated Plug-in
validation prior launching is not performed, but when that
launch-configuration is modified in the 'Run/Debug configurations'
dialog and then launched the validation happens.

The reason is that previously the JUnitWorkbenchLaunchShortcut did not
set a value for AUTOMATIC_VALIDATE when creating a new test
launch-configuration. JUnitLaunchConfigurationDelegate then used the
default of false when a that new JUnit Plug-in Test launch config was
launched (usually immediately). But when that config was opened in the
'Run/Debug configurations' dialog in
AbstractPluginBlock.initializeButtonsFrom() the check-box for 'Validate
Plug-ins automatically prior to launching' is initialized to true in
case of an absent value.

With this change AUTOMATIC_VALIDATE=false is set when creating a JUnit
Plug-in Test launch via the usual shortcut so that the validation only
happens if one activly activates it in the launch-config dialog.

Additionally set the values of some more PDE controlled attributes for
which only the default was used until now to have less changes in the
launch-config when it is opened the first time in the editor.