oeqa: Markup further tests for stdout/stderr buffering

This further cleans up the output of oe-selftest so that runqemu output
is hidden unless tests fail.

(From OE-Core rev: efa064d5026538ab513edc11869364ce2f14b977)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/selftest/cases/devtool.py

@@ -13,6 +13,8 @@ from oeqa.core.decorator.oeid import OETestID
 
 class DevtoolBase(OESelftestTestCase):
 
+    buffer = True
+
     def _test_recipe_contents(self, recipefile, checkvars, checkinherits):
         with open(recipefile, 'r') as f:
             invar = None