Don't microoptimize memory usage in tests: use separate buffers for
the input and the output. Allocate the input buffer dynamically
because the size is a parameter of the test case. Allocate the output
buffer dynamically because it's generally good practice in tests so
that a memory sanitizer can detect a buffer overflow.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>