iio: Fix sysfs attribute verification failing
sysfs usually appends newlines at the end of attributes to make them easier to consume, but the test suite can't test for that.
** (iio-sensor-proxy:4882): WARNING **: 14:14:46.792: Possible failure in string write of 'accel_3d-dev0
' Should be 'accel_3d-dev0' written to /sys/devices/platform/AMDI0010:02/i2c-2/i2c-BMA250E:00/0018:6243:0001.0002/HID-SENSOR-200073.2.auto/iio:device0/trigger/current_trigger
Closes: #347 (closed)
Fixes: ac053f8a