ktest: Allow tests to use different GRUB_MENUs

To save connecting and searching for a given grub menu for each test,
ktest.pl will cache the grub number it found. The problem is that
different tests might use a different grub menu, but ktest.pl will
ignore it.

Instead, have ktest.pl check if the grub menu it used to cache the
content is the same as when it grabbed the menu. If not, grab it again,
otherwise just return the cached value.

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