add assert_always() macro for release build assertions

test/miscellaneous/assert.cpp

@@ -0,0 +1,9 @@
+#include "../fixtures/util.hpp"
+
+#include <mbgl/util/assert.hpp>
+
+using namespace mbgl;
+
+TEST(Assert, Always) {
+    EXPECT_DEATH(assert_always(true == false), "failed assertion `true == false'");
+}