tests/legacy/kernel/test_libs: use memcpy() vs strncpy()

Coverity complained about the use of strncpy() to fill up a buffer of
size N with a string of the same size didn't leave room for the final
\0.

This is a valid concern; however, the usage is valid too, as the
writer intended to create a pattern that later can be tested--addind a
\0 would break the pattern.

So instead, use memcpy() for the same function.

Change-Id: If52d02ce41731348f4a2d750c79f9e1c51f3afcf
Coverity-ID: 151947
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>