Fix error code combination check

`mbedtls_test_err_add_check` was previously incorrectly throwing an error if
both error codes were correct and valid pure error codes. This change fixes
that behaviour to correctly throw errors when invalid combinations are found.

Signed-off-by: Chris Jones <christopher.jones@arm.com>

tests/src/helpers.c

@@ -287,7 +287,7 @@ void mbedtls_param_failed( const char *failure_condition,
 void mbedtls_test_err_add_check( int high, int low,
                                  const char *file, int line )
 {
-    if ( high < -0x0FFF && low > -0x007F )
+    if ( high > -0x1000 || low < -0x007F )
     {
         mbedtls_fprintf( stderr, "\nIncorrect error code addition at %s:%d\n",
                                 file, line );