add simple test for matrix

tests/math.cpp

@@ -0,0 +1,11 @@
+#include <gtest/gtest.h>
+
+#include "math/matrix.hpp"
+
+TEST(matrix, check)
+{
+  hack::matrix<int, 2> m { { 0, 0, 1 }, { 0, 1, 1 } };
+  m[1][1] = 123;
+
+  ASSERT_EQ(m.size(), 3);
+}