selftest/sstatetests: separate sstate presence check into its own class

This allows reusing the check function in tests other than CDN/local mirror
ones, such as sstate bundle testing.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>