testimage: Fix SDK extraction error handling

Currently if the SDK fails to extract, no error is shown and the test is marked
as passed! Clearly this is incorrect, fix it to correctly raise an error.

(From OE-Core rev: fb2235a21e45fa1a47c3b7a9a6a72c515ef10dd1)

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

meta/classes/testimage.bbclass

@@ -375,7 +375,10 @@ def testsdk_main(d):
     sdktestdir = d.expand("${WORKDIR}/testimage-sdk/")
     bb.utils.remove(sdktestdir, True)
     bb.utils.mkdirhier(sdktestdir)
-    subprocess.call("cd %s; %s <<EOF\n./tc\nY\nEOF" % (sdktestdir, tcname), shell=True)
+    try:
+        subprocess.check_output("cd %s; %s <<EOF\n./tc\nY\nEOF" % (sdktestdir, tcname), shell=True)
+    except subprocess.CalledProcessError as e:
+        bb.fatal("Couldn't install the SDK:\n%s" % e.output)
 
     try:
         targets = glob.glob(d.expand(sdktestdir + "/tc/environment-setup-*"))