plugins: fix prefs crash from null option shorttext in mock/test options

These options have null shorttext labels because they are not meant to show up in GUI preferences. They were not configured in any way to be hidden from GUI prefs though. Being available in GUI prefs yet having a null shorttext pointer breaks the expectations of the GUI prefs code resulting in a null-deref crash.

(The crash occurs upon simply selecting these plugin nodes within the advanced preferences tree, or upon opening the new upcoming 'expert' preferences interface).

The most appropriate solution is to mark these developer options as private. Since there is actually no need for these options to be saved in the saved settings file, I have marked them as volatile, which also implies private.

(This is a prerequisite for !1344 (merged)).