test_hexdump: test all possible group sizes for overflow

Currently the only one combination is tested for overflow, i.e.  rowsize =
16, groupsize = 1, len = 1.  Do various test to go through all possible
branches.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>