testexport.bbclass: use image suffix in testexport tar ball name

testexport.bbclass: use image suffix in testexport tar ball name

testexport.tar.gz is image specific. Thus add same image name
suffix to the tar ball name. For example genericarm64 machine
core-image-minimal image tar ball changes name from
testexport.tar.gz to testexport-core-image-minimal-genericarm64.tar.gz

This way testexport from multiple images can exists in the same
directory.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>