local.conf.sample: Update the sample config as per new migration manual

Syncs with oe-core and the documentation
https://www.yoctoproject.org/docs/latest/ref-manual/ref-manual.html#var-TESTIMAGE_AUTO
https://www.yoctoproject.org/docs/latest/ref-manual/ref-manual.html#migration-2.6-automatic-testing-changes

(From meta-yocto rev: 1d180bc08eb6b46be4127d0b1a067b0fb92544d5)

(From meta-yocto rev: 42ee4df294851e442d7abf11e6126c82ae3270ee)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/local.conf.sample

@@ -157,10 +157,12 @@ USER_CLASSES ?= "buildstats image-mklibs image-prelink"
 # Runtime testing of images
 #
 # The build system can test booting virtual machine images under qemu (an emulator)
-# after any root filesystems are created and run tests against those images. To
-# enable this uncomment this line. See classes/testimage(-auto).bbclass for
-# further details.
-#TEST_IMAGE = "1"
+# after any root filesystems are created and run tests against those images. It can also
+# run tests against any SDK that are built. To enable this uncomment these lines.
+# See classes/test{image,sdk}.bbclass for further details.
+#IMAGE_CLASSES += "testimage testsdk"
+#TESTIMAGE_AUTO_qemuall = "1"
+
 #
 # Interactive shell configuration
 #