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/include/test/helpers.h b/tests/include/test/helpers.h
index 2ca85d4..258c6ba 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -71,6 +71,7 @@
 void mbedtls_test_platform_teardown( void );
 
 void mbedtls_test_fail( const char *test, int line_no, const char* filename );
+void mbedtls_test_skip( const char *test, int line_no, const char* filename );
 
 /** Set the test step number for failure reports.
  *
@@ -82,7 +83,7 @@
  */
 void mbedtls_test_set_step( unsigned long step );
 
-void mbedtls_test_skip( const char *test, int line_no, const char* filename );
+void mbedtls_test_info_reset( void );
 
 /**
  * \brief          This function decodes the hexadecimal representation of