tests/heap_cmd: fix test script
tests/heap_cmd: fix test script

In c95e8553ef4761f145b51ebc9997bcfa8def48b4 the shell output of the
heap command was changed and no longer matched the expectation of the
test script. This adapts the test to again match the output.

RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT