ktest: Reset grub menu cache with different machines

Different tests may use a different machine. In such cases, we need to
try to get the current grub menu index. If the same grub menu is used
for two different machines, it may not be at the same index on the
second machine. A search for the index must be performed again.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>