test/py: fix test_efi_secboot/conftest.py

test/py: fix test_efi_secboot/conftest.py

If udisksctl is present
test/py/tests/test_efi_secboot/conftest.py
fails because the disk image is never mounted.

Normal users can only mount fuse file systems. Unfortunately fusefat is
still in an experimental state and seems not to work here correctly.

So as we have to be root or use the sudo command anyway delete all coding
referring to udisksctl.

--

We should not use mount point /mnt as this directory or one of its
sub-directories might already be in use as active mount points. Instead
create a new directory in the build root as mount point.

--

Remove debug print statements that have been commented out. print without
parentheses is anyway invalid in Python 3. And pytest anyway filters out
the output if there is no exception reported.

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