ftracetest: Convert exit -1 to exit $FAIL

POSIX says that exit takes an unsigned integer between 0 and 255, so
using -1 doesn't work on POSIX shells.

There is already a well-defined failure code, $FAIL (1), so use that.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>