classes/testimage: Add defeault inherit for testsdk.

For compatibility adds default inherit of the new class testsdk
for now, we need to review the code base.

(From OE-Core rev: a9861f6a0fd10c2a56129f2f0a7cdd806912290f)

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/testimage.bbclass

@@ -290,3 +290,5 @@ def testimage_main(d):
             target.stop()
 
 testimage_main[vardepsexclude] =+ "BB_ORIGENV"
+
+inherit testsdk