Adding test_unique_ptr_member (for desired PyCLIF behavior).

See also: https://github.com/pybind/pybind11/issues/2583

Does not build with upstream master or
https://github.com/pybind/pybind11/pull/2047, but builds with
https://github.com/RobotLocomotion/pybind11 and almost runs:

```
Running tests in directory "/usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests":
================================================================================= test session starts =================================================================================
platform linux -- Python 3.8.5, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests, inifile: pytest.ini
collected 2 items

test_unique_ptr_member.py .F                                                                                                                                                    [100%]

====================================================================================== FAILURES =======================================================================================
_____________________________________________________________________________ test_pointee_and_ptr_owner ______________________________________________________________________________

    def test_pointee_and_ptr_owner():
        obj = m.pointee()
        assert obj.get_int() == 213
        m.ptr_owner(obj)
        with pytest.raises(ValueError) as exc_info:
>           obj.get_int()
E           Failed: DID NOT RAISE <class 'ValueError'>

test_unique_ptr_member.py:17: Failed
============================================================================= 1 failed, 1 passed in 0.06s =============================================================================
```

tests/test_unique_ptr_member.cpp

@@ -0,0 +1,53 @@
+#include "pybind11_tests.h"
+
+#include <memory>
+
+namespace pybind11_tests {
+namespace unique_ptr_member {
+
+class pointee { // NOT copyable.
+  public:
+    pointee() = default;
+
+    int get_int() const { return 213; }
+
+  private:
+    pointee(const pointee &) = delete;
+    pointee(pointee &&) = delete;
+    pointee &operator=(const pointee &) = delete;
+    pointee &operator=(pointee &&) = delete;
+};
+
+class ptr_owner {
+  public:
+    explicit ptr_owner(std::unique_ptr<pointee> ptr) : ptr_(std::move(ptr)) {}
+
+  private:
+    std::unique_ptr<pointee> ptr_;
+};
+
+// Just to have a minimal example of a typical C++ pattern.
+inline int cpp_pattern() {
+    auto obj = std::unique_ptr<pointee>(new pointee);
+    int result = (obj ? 10 : 0);
+    ptr_owner owner(std::move(obj));
+    result += (obj ? 1 : 0);
+    return result;
+}
+
+TEST_SUBMODULE(unique_ptr_member, m) {
+    m.def("cpp_pattern", cpp_pattern);
+
+    py::class_<pointee>(m, "pointee")
+        .def(py::init<>())
+        .def("get_int", &pointee::get_int);
+
+    py::class_<ptr_owner>(m, "ptr_owner")
+#ifdef FEAT_UNIQUE_PTR_ARG
+        .def(py::init<std::unique_ptr<pointee>>(), py::arg("ptr"))
+#endif
+        ;
+}
+
+} // namespace unique_ptr_member
+} // namespace pybind11_tests

tests/test_unique_ptr_member.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+import pytest
+
+from pybind11_tests import unique_ptr_member as m
+
+
+def test_cpp_pattern():
+    res = m.cpp_pattern()
+    assert res == 10
+
+
+def test_pointee_and_ptr_owner():
+    obj = m.pointee()
+    assert obj.get_int() == 213
+    m.ptr_owner(obj)
+    with pytest.raises(ValueError) as exc_info:
+        obj.get_int()
+    assert str(exc_info.value).startswith("Missing value for wrapped C++ type ")