test: adjust names of Unicode test functions

In test/py/conftest.py the assumption is made that for if a test is called
with `ut unicode` the test function name starts with 'unicode_test_'. As
the Unicode tests did not follow this naming scheme they were not executed
by `make tests`.

Rename the Unicode test functions.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>