test_hexdump: define FILL_CHAR constant

Define a character to fill the test buffers.  Though the character should
be printable since it's used when errors are reported.  It should neither
be from hex digit [a-fA-F0-9] dictionary nor space.  It is recommended not
to use one which is present in ASCII part of the test data.  Later on we
might switch to unprintable character to make test case more robust.

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