test: suppress gcc format warnings.

On a Mint 13 32-bit OS, this warning is produced on build:

src/tests.c: In function ‘printElement’:
src/tests.c:22:3: warning: format ‘%ld’ expects argument of type ‘long
int’, but argument 2 has type ‘uint64_t’ [-Wformat]

Using the arch independent format string suppresses the warning message.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>