lib/test_string.c: add some testcases for strchr and strnchr

Make sure that the trailing NUL is considered part of the string and can
be found.

Link: http://lkml.kernel.org/r/20190506124634.6807-4-peda@axentia.se


Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>