Add macro for error code addition

Adds a macro (`MBEDTLS_ERR_ADD`) to add error codes together and check that the
result will not be corrupted. This additional check is only enabled during
testing when `MBEDTLS_TEST_HOOKS` is defined.

Also includes a reference usage example in `rsa.c` where two high-level error
codes could be incorrectly added together under the right conditions. This now
ensures that when this error occurs during testing it will be correctly
reported.

Signed-off-by: Chris Jones <christopher.jones@arm.com>

tests/suites/main_test.function

@@ -33,6 +33,10 @@
 #include "psa/crypto.h"
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+#include "mbedtls/error.h"
+#endif
+
 /* Test code may use deprecated identifiers only if the preprocessor symbol
  * MBEDTLS_TEST_DEPRECATED is defined. When building tests, set
  * MBEDTLS_TEST_DEPRECATED explicitly if MBEDTLS_DEPRECATED_WARNING is
@@ -279,6 +283,10 @@ $platform_code
  */
 int main( int argc, const char *argv[] )
 {
+#if defined(MBEDTLS_TEST_HOOKS)
+    mbedtls_test_err_add_hook = &mbedtls_test_err_add_check;
+#endif
+
     int ret = mbedtls_test_platform_setup();
     if( ret != 0 )
     {