Switch to integer data type in Eigen test for default arguments.

The `test10_eigen_scalar_default` test in `test_eigen.py` relies
unnecessarily on exact equality of floating point numbers. In my
environment, with a recent commit of Eigen, the addition in the test is
causing the equality check to fail. There might be other upstream issues
to consider here, but the specific test failure here seems to be a false
positive! This PR switches that test to use integer zeros to be less
sensitive to numerics. Let me know what you think - thanks!

tests/test_eigen.cpp

@@ -169,7 +169,7 @@ NB_MODULE(test_eigen_ext, m) {
     m.def("sparse_complex", []() -> Eigen::SparseMatrix<std::complex<double>> { return {}; });
 
     /// issue #166
-    using Matrix1d = Eigen::Matrix<double,1,1>;
+    using Matrix1d = Eigen::Matrix<int,1,1>;
     try {
         m.def(
             "default_arg", [](Matrix1d a, Matrix1d b) { return a + b; },