Add `mbedtls_` prefix to all public names in `helpers.h`

Adds the `mbedtls_` prefix to `test_result_t` and `test_info` and updates
any references to them. This is to follow the naming convention as these are
now declared in a public namespace.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 8d31048..2ca85d4 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -51,21 +51,21 @@
 
 typedef enum
 {
-    TEST_RESULT_SUCCESS = 0,
-    TEST_RESULT_FAILED,
-    TEST_RESULT_SKIPPED
-} test_result_t;
+    MBEDTLS_TEST_RESULT_SUCCESS = 0,
+    MBEDTLS_TEST_RESULT_FAILED,
+    MBEDTLS_TEST_RESULT_SKIPPED
+} mbedtls_test_result_t;
 
 typedef struct
 {
-    test_result_t result;
+    mbedtls_test_result_t result;
     const char *test;
     const char *filename;
     int line_no;
     unsigned long step;
 }
-test_info_t;
-extern test_info_t test_info;
+mbedtls_test_info_t;
+extern mbedtls_test_info_t mbedtls_test_info;
 
 int mbedtls_test_platform_setup( void );
 void mbedtls_test_platform_teardown( void );