tests/tinycrypt/aes: Remove struct declaration inside function

This commit removes the declaration of a data type inside the tests
3 & 4. The new data type 'struct kat_table' is declared at the
beginning of the file.

Change-Id: I7662cd164a865fe0e70f984b9b494d96b4a72af9
Signed-off-by: Flavio Santes <flavio.santes@intel.com>