Fix ODR violations in our Eigen Tensor tests (#4412)

* First

* Fix centos 7 again :(

* Fix minor nits

tests/eigen_tensor_avoid_stl_array.cpp

@@ -0,0 +1,14 @@
+/*
+    tests/eigen_tensor.cpp -- automatic conversion of Eigen Tensor
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#ifndef EIGEN_AVOID_STL_ARRAY
+#    define EIGEN_AVOID_STL_ARRAY
+#endif
+
+#include "test_eigen_tensor.inl"
+
+PYBIND11_MODULE(eigen_tensor_avoid_stl_array, m) { eigen_tensor_test::test_module(m); }