ctests: utf8-plain now works with all export variants.

ctests: utf8-plain now works with all export variants.

utf8-plain (Unicode (utf8 XeTeX)) is a power-user setting
for the input encoding with two use cases:

a) setup of system fonts or

b) setup of input encoding supportuser preamble

in the document class or user preamble.

The test file is an example for use case b.