Print to System.err in test 909 to avoid output races.

Throwable.printStackTrace() prints to System.err, so print
all other output to System.err as well to avoid races like

    #################### info
    # Tests jvmti plugin attaching during live phase.
    #################### diffs
    --- expected.txt	2017-01-13 18:22:13.897224469 +0000
    +++ output.txt	2017-01-30 17:51:21.091550290 +0000
    @@ -6,6 +6,6 @@
     Goodbye!
     Hello, world!
     java.io.IOException: Process is not debuggable.
    +Goodbye!
     	at dalvik.system.VMDebug.attachAgent(Native Method)
     	at Main.main(Main.java:27)
    -Goodbye!
    ####################

Test: m test-art-host-run-test-909-attach-agent
Change-Id: I956bd24db056154b938568c156df80d4ff56ba11