selftest: do not hardcode /tmp/sdk

This races if there are several copies of the test running at the same
time.

[YOCTO #14438]

(From OE-Core rev: deab11848036941771f2b3dc5cdaee83395280b5)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>