lib/test_string.c: move string selftest in the Runtime Testing menu

STRING_SELFTEST is presented in the "Library routines" menu.  Move it in
Kernel hacking > Kernel Testing and Coverage > Runtime Testing together
with other similar tests found in lib/

	--- Runtime Testing
	<*>   Test functions located in the hexdump module at runtime
	<*>   Test string functions (NEW)
	<*>   Test functions located in the string_helpers module at runtime
	<*>   Test strscpy*() family of functions at runtime
	<*>   Test kstrto*() family of functions at runtime
	<*>   Test printf() family of functions at runtime
	<*>   Test scanf() family of functions at runtime

Link: https://lkml.kernel.org/r/20210719185158.190371-1-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Peter Rosin <peda@axentia.se>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>