gallium/aux/util: use util_snprintf() in test_texture_barrier

Instead of plain snprintf(). To fix the MSVC 2013 build:

  Compiling src\gallium\auxiliary\util\u_tests.c ...
u_tests.c
src\gallium\auxiliary\util\u_tests.c(624) : warning C4013: 'snprintf' undefined; assuming extern returning int

...

gallium.lib(u_tests.obj) : error LNK2019: unresolved external symbol _snprintf referenced in function _test_texture_barrier
build\windows-x86-debug\gallium\targets\graw-gdi\graw.dll : fatal error LNK1120: 1 unresolved externals
scons: *** [build\windows-x86-debug\gallium\targets\graw-gdi\graw.dll] Error 1120
scons: building terminated because of errors.

Fixes: 56342c97 ("gallium/u_tests: test FBFETCH and shader-based blending with MSAA")
Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Brian Paul <brianp@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: Dieter Nützel <Dieter@nuetzel-hh.de>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Brian Paul <brianp@vmware.com>