selftest: devtool: Separate common functions and devtool sstate setup into two classes

The selftest recipetool base class reuse the selftest devtool base
class. Thereby the selftest devtool base class setup its own devtool
sstate and the selftest recipetool classes trigger the build of recipes.
This leads to the problem that the build artifacts doesn't reach the
persistent sstate cache and rebuild on every selftest run.

Move the common selftest devtool functions into its own class and use
the sstate cache in the recipetool tests.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>