ktest: introduce _get_grub_index

Introduce _get_grub_index() to deal with Boot Loader
Specification (BLS) and cleanup.

Link: http://lkml.kernel.org/r/20190509213647.6276-2-msys.mizuma@gmail.com



Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>