snprintf: add buffer tests

Add some tests that test the buffer size limitation checks that snprintf
should implement.
This tests corner cases of the buffer size (tight buffer, one byte too
small, etc.), also the proper cutoff and return values if the buffer is
not sufficient.

Change-Id: Ia3d6309cfc8624904f68fe185bcaa91ec517c7b4
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
1 file changed