If "snapshot" special file doesn't exist, that kernel does
not support snapshot and snapshot trigger too. In that case
snapshot trigger test results to unsupported instead of fail.

Link: http://lkml.kernel.org/r/147928401215.22982.10411665829041109794.stgit@devboxSigned-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>