Use named constants FROM_PAIR/FROM_PUBLIC for readability

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_pk.function b/tests/suites/test_suite_pk.function
index 3ce7d26..52d0f68 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -313,6 +313,12 @@
 }
 #endif /* MBEDTLS_RSA_C */
 
+typedef enum {
+    /* The values are compatible with thinking of "from pair" as a boolean. */
+    FROM_PUBLIC = 0,
+    FROM_PAIR = 1
+} from_pair_t;
+
 #if defined(MBEDTLS_PSA_CRYPTO_C)
 static int pk_setup_for_type(mbedtls_pk_type_t pk_type, int want_pair,
                              mbedtls_pk_context *pk, psa_key_type_t *psa_type)