Remove direct writing to `test_info` from `*.function`

Add a new function `mbedtls_test_info_reset()` to remove direct writes to
`mbedtls_test_info`. This change still allows values to be read directly
however all writes are now done inside of `helpers.c`.

Also slightly reordered code to make it easier to read.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 9bfd7e0..e323275 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -93,11 +93,6 @@
     mbedtls_test_info.filename = filename;
 }
 
-void mbedtls_test_set_step( unsigned long step )
-{
-    mbedtls_test_info.step = step;
-}
-
 void mbedtls_test_skip( const char *test, int line_no, const char* filename )
 {
     mbedtls_test_info.result = MBEDTLS_TEST_RESULT_SKIPPED;
@@ -106,6 +101,20 @@
     mbedtls_test_info.filename = filename;
 }
 
+void mbedtls_test_set_step( unsigned long step )
+{
+    mbedtls_test_info.step = step;
+}
+
+void mbedtls_test_info_reset( void )
+{
+    mbedtls_test_info.result = MBEDTLS_TEST_RESULT_SUCCESS;
+    mbedtls_test_info.step = (unsigned long)( -1 );
+    mbedtls_test_info.test = 0;
+    mbedtls_test_info.line_no = 0;
+    mbedtls_test_info.filename = 0;
+}
+
 int mbedtls_test_unhexify( unsigned char *obuf,
                            size_t obufmax,
                            const char *ibuf,