oeqa/selftest: Use SSTATE_DIR of parent build dir

This will configure the build-st/conf/local.conf with the SSTATE_DIR
of the parent build dir to speed up the build, test, dev process.

Signed-off-by: Thomas Roos <throos@amazon.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>