lib/: make RUNTIME_TESTS a menuconfig to ease disabling it all

No need to get into the submenu to disable all related config entries.

This makes it easier to disable all RUNTIME_TESTS config options without
entering the submenu.  It will also enable one to see that en/dis-abled
state from the outside menu.

This is only intended to change menuconfig UI, not change the config
dependencies.

Link: http://lkml.kernel.org/r/20171209162742.7363-1-vincent.legoll@gmail.com
Signed-off-by: Vincent Legoll <vincent.legoll@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.debug

@@ -1641,7 +1641,10 @@ config DMA_API_DEBUG
 
 	  If unsure, say N.
 
-menu "Runtime Testing"
+menuconfig RUNTIME_TESTING_MENU
+	bool "Runtime Testing"
+
+if RUNTIME_TESTING_MENU
 
 config LKDTM
 	tristate "Linux Kernel Dump Test Tool Module"
@@ -1929,7 +1932,7 @@ config TEST_DEBUG_VIRTUAL
 
 	  If unsure, say N.
 
-endmenu # runtime tests
+endif # RUNTIME_TESTING_MENU
 
 config MEMTEST
 	bool "Memtest"