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

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