support/testing: rename variables for readability

The 'lines' variable is overwritten with its own fields. Thus it
contains a line first, and then a list of fields -- it never contains
'lines'.

Use two different variables named 'line' and 'fields' to make the code
more readable.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>