ftracetest: Fix to show descriptions on dash

The ftracetest doesn't show testcase's descriptions when
it is executed on dash. This fixes that to show the
descriptions on dash correctly by passing it via a variable
instead of directly passing the grep command output.

Link: http://lkml.kernel.org/r/20141204194116.7376.78940.stgit@localhost.localdomain
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>