nixos/test-driver/Logger: Replace invalid UTF-8

Regression introduced by d84741a4.

The mentioned commit actually is a good thing, because we now get the
output from the X session.

Unfortunately, for the i3wm test, the i3-config-wizard prints out the
raw keyboard symbols directly coming from xcb, so the output isn't
necessarily proper UTF-8.

As the XML::Writer already expects valid UTF-8 input, we assume that
everything that comes into sanitise() will be UTF-8 from the start. So
we just decode() it using FB_DEFAULT as the check argument so that
every invalid character is replaced by the unicode replacement
character:

https://en.wikipedia.org/wiki/Specials_(Unicode_block)#Replacement_character

We simply re-oncode it again afterwards and return it, so we should
always get out valid UTF-8 in the log XML.

For more information about FB_DEFAULT and FB_CROAK, have a look at:

http://search.cpan.org/~dankogai/Encode-2.84/Encode.pm#Handling_Malformed_Data



Signed-off-by: aszlig <aszlig@redmoonstudios.org>