oeqa: avoid class setup method to run when skipping the whole class

For now, even if we have specified to skip the whole module/class via
command line, e.g., `oe-selftest -R gotoolchain', the class setup method
is still run. This at least results in unnecessary builds, and at worst
results in ERROR, if the setup method fails.

So improve the skipping mechanism to avoid class setup method to run
when specified to skip.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>