cmd_ut: add a parameter prefix to the function cmd_ut_category

There is black magic in the file conftest.py that list
all the test unit. Then, all those test unit are called
in pytest. This call is done with the end of the name
(for example checksum if the full name is bloblist_test_checksum).

The result is that only test for dm are really executed.
by pytest, all others tests are listed but never executed.

This behaviour happens because the dm test unit only check
the end of the name and others tests checks the full name.

To fix this issue, I've added a prefix to the function
cmd_ut_category, and this prefix is removed when looking
for the unit test.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>