oeqa/selftest/oescripts: refactor skipping logic

oeqa/selftest/oescripts: refactor skipping logic

OEScriptTests currently skips if cairo isn't present, and does a build
of core-image-minimal.  This is only required for the
OEPybootchartguyTests tests, so move that logic there so that the
OEListPackageconfigTests run even if cairo isn't available.

This leaves OEScriptTests as a simple class containing the scripts_dir
assignment, which can then be reused by other tests to avoid code
duplication.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>