In `test_suite_debug`, test `mbedtls_debug_snprintf()`, which uses
`mbedtls_vsnprintf()` like `mbedtls_debug_print_msg()`. Do this instead of
testing `mbedtls_snprintf()`, which might be subtly different (older
Windows runtimes had slightly different behavior for vsnprintf() vs
snprintf(); TF-PSA-Crypto might pick up a different function if the
platform configuration is different in TF-PSA-Crypto and Mbed TLS).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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