test: add automation framework and sanitytest

Automation test is disabled by default. User need set TESTCLASS
to qemu in conf/local.conf and run bitbake command "bitbake
poky-image-xxx" or "bitbake poky-image-xxx -c qemuimagetest" to
trigger it. Currently only the sanity test with two testcases are
added.

To run the test, user need prepare a testing environment:
1) "expect" should be installed on system
2) NOPASSWD should be set for user to run bitbake

Signed-off-by Jiajun Xu <jiajun.xu@intel.com>

meta/classes/image.bbclass

@@ -1,5 +1,8 @@
 inherit rootfs_${IMAGE_PKGTYPE}
 
+IMAGETEST ?= "dummy"
+inherit imagetest-${IMAGETEST}
+
 LICENSE = "MIT"
 PACKAGES = ""
 RDEPENDS += "${IMAGE_INSTALL}"