Fix memory leak in test_suite_x509write with PSA crypto

The documentation of mbedtls_pk_wrap_as_opaque is quite clear:

 * \param handle    Output: a PSA key handle.
 *                  It's the caller's responsibility to call
 *                  psa_destroy_key() on that handle after calling
 *                  mbedtls_pk_free() on the PK context.

But the test failed to call psa_destroy_key().

While at it, also use PSA_DONE(): it ensures that if we fail to destroy the
key, we'll get an explicit error message about it without the need for
valgrind.

This is a preliminary to adding a valgrind-based test for constant-flow code:
we need to make sure the rest of the tests are fully valgrind-clean, which
they weren't.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index be9e0ae..b205b74 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -5,12 +5,20 @@
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
 #include "mbedtls/rsa.h"
+
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
 #include "psa/crypto.h"
 #include "mbedtls/psa_util.h"
+#include "test/psa_crypto_helpers.h"
+#define PSA_INIT( ) PSA_ASSERT( psa_crypto_init( ) )
+#else
+/* Define empty macros so that we can use them in the preamble and teardown
+ * of every test function that uses PSA conditionally based on
+ * MBEDTLS_USE_PSA_CRYPTO. */
+#define PSA_INIT( ) ( (void) 0 )
+#define PSA_DONE( ) ( (void) 0 )
 #endif
 
-
 #if defined(MBEDTLS_RSA_C)
 int mbedtls_rsa_decrypt_func( void *ctx, int mode, size_t *olen,
                        const unsigned char *input, unsigned char *output,
@@ -156,7 +164,7 @@
     const char *subject_name = "C=NL,O=PolarSSL,CN=PolarSSL Server 1";
     mbedtls_test_rnd_pseudo_info rnd_info;
 
-    psa_crypto_init();
+    PSA_INIT( );
     memset( &rnd_info, 0x2a, sizeof( mbedtls_test_rnd_pseudo_info ) );
 
     md_alg_psa = mbedtls_psa_translate_md( (mbedtls_md_type_t) md_type );
@@ -184,9 +192,12 @@
     buf[pem_len] = '\0';
     TEST_ASSERT( x509_crt_verifycsr( buf, pem_len + 1 ) == 0 );
 
+
 exit:
     mbedtls_x509write_csr_free( &req );
     mbedtls_pk_free( &key );
+    psa_destroy_key( slot );
+    PSA_DONE( );
 }
 /* END_CASE */