test/hello_world: Properly report P9MAMBO path

In the P9 hello script we look for $P9MAMBO_BINARY, but then if we don't
find it we print a message using $MAMBO_BINARY, which is confusing:

  $ set -x P9MAMBO_BINARY run/p9/run_cmdline
  $ ./test/hello_world/run_mambo_p9_hello_world.sh
  Could not find executable P9MAMBO_BINARY (/opt/ibm/systemsim-p9//).

Fix it so the right thing is printed:

  Could not find executable P9MAMBO_BINARY (/opt/ibm/systemsim-p9//run/p9/run_cmdline).

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

test/hello_world/run_mambo_p9_hello_world.sh

@@ -9,7 +9,7 @@ if [ -z "$P9MAMBO_BINARY" ]; then
 fi
 
 if [ ! -x "$P9MAMBO_PATH/$P9MAMBO_BINARY" ]; then
-    echo "Could not find executable P9MAMBO_BINARY ($P9MAMBO_PATH/$MAMBO_BINARY). Skipping hello_world test";
+    echo "Could not find executable P9MAMBO_BINARY ($P9MAMBO_PATH/$P9MAMBO_BINARY). Skipping hello_world test";
     exit 0;
 fi