For iOS, the LibreOffice unit tests that normally are built as
"plugins", i.e. dynamic libraries, are actually built as static
libraries, because we don't do any dynamic linking of own code on iOS.

Enable giving the "plugin" unit test entry point a different name than
a fixed "cppunitTestPlugIn", so that several unit tests can be linked
together into one executable, for simplicity.

Drop the stupid dummy main() function in cppunit plugins. I doubt any
Unix platform we care for needs it, but drop just for iOS and MacOSX
for now.