kernel-hacking: move kernel testing and coverage options to same submenu

Move error injection, coverage, testing options to a new top level
submenu 'Kernel Testing and Coverage'.  They are all for test purpose.

Link: http://lkml.kernel.org/r/20190909144453.3520-5-changbin.du@gmail.com


Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>